Merge commit '9bba04fed2291aa884d1da64bc62150d5562cc5b'

[unleashed.git] / kernel / fs / zfs / metaslab.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2011, 2015 by Delphix. All rights reserved.
 * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
 * Copyright (c) 2014 Integros [integros.com]
 */

#include <sys/zfs_context.h>
#include <sys/dmu.h>
#include <sys/dmu_tx.h>
#include <sys/space_map.h>
#include <sys/metaslab_impl.h>
#include <sys/vdev_impl.h>
#include <sys/zio.h>
#include <sys/spa_impl.h>
#include <sys/zfeature.h>
#include <sys/vdev_indirect_mapping.h>
#include <sys/zap.h>

#define GANG_ALLOCATION(flags) \
        ((flags) & (METASLAB_GANG_CHILD | METASLAB_GANG_HEADER))

uint64_t metaslab_aliquot = 512ULL << 10;
uint64_t metaslab_force_ganging = SPA_MAXBLOCKSIZE + 1; /* force gang blocks */

/*
 * Since we can touch multiple metaslabs (and their respective space maps)
 * with each transaction group, we benefit from having a smaller space map
 * block size since it allows us to issue more I/O operations scattered
 * around the disk.
 */
int zfs_metaslab_sm_blksz = (1 << 12);

/*
 * The in-core space map representation is more compact than its on-disk form.
 * The zfs_condense_pct determines how much more compact the in-core
 * space map representation must be before we compact it on-disk.
 * Values should be greater than or equal to 100.
 */
int zfs_condense_pct = 200;

/*
 * Condensing a metaslab is not guaranteed to actually reduce the amount of
 * space used on disk. In particular, a space map uses data in increments of
 * MAX(1 << ashift, space_map_blksize), so a metaslab might use the
 * same number of blocks after condensing. Since the goal of condensing is to
 * reduce the number of IOPs required to read the space map, we only want to
 * condense when we can be sure we will reduce the number of blocks used by the
 * space map. Unfortunately, we cannot precisely compute whether or not this is
 * the case in metaslab_should_condense since we are holding ms_lock. Instead,
 * we apply the following heuristic: do not condense a spacemap unless the
 * uncondensed size consumes greater than zfs_metaslab_condense_block_threshold
 * blocks.
 */
int zfs_metaslab_condense_block_threshold = 4;

/*
 * The zfs_mg_noalloc_threshold defines which metaslab groups should
 * be eligible for allocation. The value is defined as a percentage of
 * free space. Metaslab groups that have more free space than
 * zfs_mg_noalloc_threshold are always eligible for allocations. Once
 * a metaslab group's free space is less than or equal to the
 * zfs_mg_noalloc_threshold the allocator will avoid allocating to that
 * group unless all groups in the pool have reached zfs_mg_noalloc_threshold.
 * Once all groups in the pool reach zfs_mg_noalloc_threshold then all
 * groups are allowed to accept allocations. Gang blocks are always
 * eligible to allocate on any metaslab group. The default value of 0 means
 * no metaslab group will be excluded based on this criterion.
 */
int zfs_mg_noalloc_threshold = 0;

/*
 * Metaslab groups are considered eligible for allocations if their
 * fragmenation metric (measured as a percentage) is less than or equal to
 * zfs_mg_fragmentation_threshold. If a metaslab group exceeds this threshold
 * then it will be skipped unless all metaslab groups within the metaslab
 * class have also crossed this threshold.
 */
int zfs_mg_fragmentation_threshold = 85;

/*
 * Allow metaslabs to keep their active state as long as their fragmentation
 * percentage is less than or equal to zfs_metaslab_fragmentation_threshold. An
 * active metaslab that exceeds this threshold will no longer keep its active
 * status allowing better metaslabs to be selected.
 */
int zfs_metaslab_fragmentation_threshold = 70;

/*
 * When set will load all metaslabs when pool is first opened.
 */
int metaslab_debug_load = 0;

/*
 * When set will prevent metaslabs from being unloaded.
 */
int metaslab_debug_unload = 0;

/*
 * Minimum size which forces the dynamic allocator to change
 * it's allocation strategy.  Once the space map cannot satisfy
 * an allocation of this size then it switches to using more
 * aggressive strategy (i.e search by size rather than offset).
 */
uint64_t metaslab_df_alloc_threshold = SPA_OLD_MAXBLOCKSIZE;

/*
 * The minimum free space, in percent, which must be available
 * in a space map to continue allocations in a first-fit fashion.
 * Once the space map's free space drops below this level we dynamically
 * switch to using best-fit allocations.
 */
int metaslab_df_free_pct = 4;

/*
 * A metaslab is considered "free" if it contains a contiguous
 * segment which is greater than metaslab_min_alloc_size.
 */
uint64_t metaslab_min_alloc_size = DMU_MAX_ACCESS;

/*
 * Percentage of all cpus that can be used by the metaslab taskq.
 */
int metaslab_load_pct = 50;

/*
 * Determines how many txgs a metaslab may remain loaded without having any
 * allocations from it. As long as a metaslab continues to be used we will
 * keep it loaded.
 */
int metaslab_unload_delay = TXG_SIZE * 2;

/*
 * Max number of metaslabs per group to preload.
 */
int metaslab_preload_limit = SPA_DVAS_PER_BP;

/*
 * Enable/disable preloading of metaslab.
 */
boolean_t metaslab_preload_enabled = B_TRUE;

/*
 * Enable/disable fragmentation weighting on metaslabs.
 */
boolean_t metaslab_fragmentation_factor_enabled = B_TRUE;

/*
 * Enable/disable lba weighting (i.e. outer tracks are given preference).
 */
boolean_t metaslab_lba_weighting_enabled = B_TRUE;

/*
 * Enable/disable metaslab group biasing.
 */
boolean_t metaslab_bias_enabled = B_TRUE;

/*
 * Enable/disable remapping of indirect DVAs to their concrete vdevs.
 */
boolean_t zfs_remap_blkptr_enable = B_TRUE;

/*
 * Enable/disable segment-based metaslab selection.
 */
boolean_t zfs_metaslab_segment_weight_enabled = B_TRUE;

/*
 * When using segment-based metaslab selection, we will continue
 * allocating from the active metaslab until we have exhausted
 * zfs_metaslab_switch_threshold of its buckets.
 */
int zfs_metaslab_switch_threshold = 2;

/*
 * Internal switch to enable/disable the metaslab allocation tracing
 * facility.
 */
boolean_t metaslab_trace_enabled = B_TRUE;

/*
 * Maximum entries that the metaslab allocation tracing facility will keep
 * in a given list when running in non-debug mode. We limit the number
 * of entries in non-debug mode to prevent us from using up too much memory.
 * The limit should be sufficiently large that we don't expect any allocation
 * to every exceed this value. In debug mode, the system will panic if this
 * limit is ever reached allowing for further investigation.
 */
uint64_t metaslab_trace_max_entries = 5000;

static uint64_t metaslab_weight(metaslab_t *);
static void metaslab_set_fragmentation(metaslab_t *);
static void metaslab_free_impl(vdev_t *, uint64_t, uint64_t, boolean_t);
static void metaslab_check_free_impl(vdev_t *, uint64_t, uint64_t);

kmem_cache_t *metaslab_alloc_trace_cache;

/*
 * ==========================================================================
 * Metaslab classes
 * ==========================================================================
 */
metaslab_class_t *
metaslab_class_create(spa_t *spa, metaslab_ops_t *ops)
{
        metaslab_class_t *mc;

        mc = kmem_zalloc(sizeof (metaslab_class_t), KM_SLEEP);

        mc->mc_spa = spa;
        mc->mc_rotor = NULL;
        mc->mc_ops = ops;
        mutex_init(&mc->mc_lock, NULL, MUTEX_DEFAULT, NULL);
        refcount_create_tracked(&mc->mc_alloc_slots);

        return (mc);
}

void
metaslab_class_destroy(metaslab_class_t *mc)
{
        ASSERT(mc->mc_rotor == NULL);
        ASSERT(mc->mc_alloc == 0);
        ASSERT(mc->mc_deferred == 0);
        ASSERT(mc->mc_space == 0);
        ASSERT(mc->mc_dspace == 0);

        refcount_destroy(&mc->mc_alloc_slots);
        mutex_destroy(&mc->mc_lock);
        kmem_free(mc, sizeof (metaslab_class_t));
}

int
metaslab_class_validate(metaslab_class_t *mc)
{
        metaslab_group_t *mg;
        vdev_t *vd;

        /*
         * Must hold one of the spa_config locks.
         */
        ASSERT(spa_config_held(mc->mc_spa, SCL_ALL, RW_READER) ||
            spa_config_held(mc->mc_spa, SCL_ALL, RW_WRITER));

        if ((mg = mc->mc_rotor) == NULL)
                return (0);

        do {
                vd = mg->mg_vd;
                ASSERT(vd->vdev_mg != NULL);
                ASSERT3P(vd->vdev_top, ==, vd);
                ASSERT3P(mg->mg_class, ==, mc);
                ASSERT3P(vd->vdev_ops, !=, &vdev_hole_ops);
        } while ((mg = mg->mg_next) != mc->mc_rotor);

        return (0);
}

void
metaslab_class_space_update(metaslab_class_t *mc, int64_t alloc_delta,
    int64_t defer_delta, int64_t space_delta, int64_t dspace_delta)
{
        atomic_add_64(&mc->mc_alloc, alloc_delta);
        atomic_add_64(&mc->mc_deferred, defer_delta);
        atomic_add_64(&mc->mc_space, space_delta);
        atomic_add_64(&mc->mc_dspace, dspace_delta);
}

uint64_t
metaslab_class_get_alloc(metaslab_class_t *mc)
{
        return (mc->mc_alloc);
}

uint64_t
metaslab_class_get_deferred(metaslab_class_t *mc)
{
        return (mc->mc_deferred);
}

uint64_t
metaslab_class_get_space(metaslab_class_t *mc)
{
        return (mc->mc_space);
}

uint64_t
metaslab_class_get_dspace(metaslab_class_t *mc)
{
        return (spa_deflate(mc->mc_spa) ? mc->mc_dspace : mc->mc_space);
}

void
metaslab_class_histogram_verify(metaslab_class_t *mc)
{
        vdev_t *rvd = mc->mc_spa->spa_root_vdev;
        uint64_t *mc_hist;
        int i;

        if ((zfs_flags & ZFS_DEBUG_HISTOGRAM_VERIFY) == 0)
                return;

        mc_hist = kmem_zalloc(sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE,
            KM_SLEEP);

        for (int c = 0; c < rvd->vdev_children; c++) {
                vdev_t *tvd = rvd->vdev_child[c];
                metaslab_group_t *mg = tvd->vdev_mg;

                /*
                 * Skip any holes, uninitialized top-levels, or
                 * vdevs that are not in this metalab class.
                 */
                if (!vdev_is_concrete(tvd) || tvd->vdev_ms_shift == 0 ||
                    mg->mg_class != mc) {
                        continue;
                }

                for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
                        mc_hist[i] += mg->mg_histogram[i];
        }

        for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
                VERIFY3U(mc_hist[i], ==, mc->mc_histogram[i]);

        kmem_free(mc_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
}

/*
 * Calculate the metaslab class's fragmentation metric. The metric
 * is weighted based on the space contribution of each metaslab group.
 * The return value will be a number between 0 and 100 (inclusive), or
 * ZFS_FRAG_INVALID if the metric has not been set. See comment above the
 * zfs_frag_table for more information about the metric.
 */
uint64_t
metaslab_class_fragmentation(metaslab_class_t *mc)
{
        vdev_t *rvd = mc->mc_spa->spa_root_vdev;
        uint64_t fragmentation = 0;

        spa_config_enter(mc->mc_spa, SCL_VDEV, FTAG, RW_READER);

        for (int c = 0; c < rvd->vdev_children; c++) {
                vdev_t *tvd = rvd->vdev_child[c];
                metaslab_group_t *mg = tvd->vdev_mg;

                /*
                 * Skip any holes, uninitialized top-levels,
                 * or vdevs that are not in this metalab class.
                 */
                if (!vdev_is_concrete(tvd) || tvd->vdev_ms_shift == 0 ||
                    mg->mg_class != mc) {
                        continue;
                }

                /*
                 * If a metaslab group does not contain a fragmentation
                 * metric then just bail out.
                 */
                if (mg->mg_fragmentation == ZFS_FRAG_INVALID) {
                        spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
                        return (ZFS_FRAG_INVALID);
                }

                /*
                 * Determine how much this metaslab_group is contributing
                 * to the overall pool fragmentation metric.
                 */
                fragmentation += mg->mg_fragmentation *
                    metaslab_group_get_space(mg);
        }
        fragmentation /= metaslab_class_get_space(mc);

        ASSERT3U(fragmentation, <=, 100);
        spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
        return (fragmentation);
}

/*
 * Calculate the amount of expandable space that is available in
 * this metaslab class. If a device is expanded then its expandable
 * space will be the amount of allocatable space that is currently not
 * part of this metaslab class.
 */
uint64_t
metaslab_class_expandable_space(metaslab_class_t *mc)
{
        vdev_t *rvd = mc->mc_spa->spa_root_vdev;
        uint64_t space = 0;

        spa_config_enter(mc->mc_spa, SCL_VDEV, FTAG, RW_READER);
        for (int c = 0; c < rvd->vdev_children; c++) {
                uint64_t tspace;
                vdev_t *tvd = rvd->vdev_child[c];
                metaslab_group_t *mg = tvd->vdev_mg;

                if (!vdev_is_concrete(tvd) || tvd->vdev_ms_shift == 0 ||
                    mg->mg_class != mc) {
                        continue;
                }

                /*
                 * Calculate if we have enough space to add additional
                 * metaslabs. We report the expandable space in terms
                 * of the metaslab size since that's the unit of expansion.
                 * Adjust by efi system partition size.
                 */
                tspace = tvd->vdev_max_asize - tvd->vdev_asize;
                if (tspace > mc->mc_spa->spa_bootsize) {
                        tspace -= mc->mc_spa->spa_bootsize;
                }
                space += P2ALIGN(tspace, 1ULL << tvd->vdev_ms_shift);
        }
        spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
        return (space);
}

static int
metaslab_compare(const void *x1, const void *x2)
{
        const metaslab_t *m1 = x1;
        const metaslab_t *m2 = x2;

        if (m1->ms_weight < m2->ms_weight)
                return (1);
        if (m1->ms_weight > m2->ms_weight)
                return (-1);

        /*
         * If the weights are identical, use the offset to force uniqueness.
         */
        if (m1->ms_start < m2->ms_start)
                return (-1);
        if (m1->ms_start > m2->ms_start)
                return (1);

        ASSERT3P(m1, ==, m2);

        return (0);
}

/*
 * Verify that the space accounting on disk matches the in-core range_trees.
 */
void
metaslab_verify_space(metaslab_t *msp, uint64_t txg)
{
        spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
        uint64_t allocated = 0;
        uint64_t sm_free_space, msp_free_space;

        ASSERT(MUTEX_HELD(&msp->ms_lock));

        if ((zfs_flags & ZFS_DEBUG_METASLAB_VERIFY) == 0)
                return;

        /*
         * We can only verify the metaslab space when we're called
         * from syncing context with a loaded metaslab that has an allocated
         * space map. Calling this in non-syncing context does not
         * provide a consistent view of the metaslab since we're performing
         * allocations in the future.
         */
        if (txg != spa_syncing_txg(spa) || msp->ms_sm == NULL ||
            !msp->ms_loaded)
                return;

        sm_free_space = msp->ms_size - space_map_allocated(msp->ms_sm) -
            space_map_alloc_delta(msp->ms_sm);

        /*
         * Account for future allocations since we would have already
         * deducted that space from the ms_freetree.
         */
        for (int t = 0; t < TXG_CONCURRENT_STATES; t++) {
                allocated +=
                    range_tree_space(msp->ms_allocating[(txg + t) & TXG_MASK]);
        }

        msp_free_space = range_tree_space(msp->ms_allocatable) + allocated +
            msp->ms_deferspace + range_tree_space(msp->ms_freed);

        VERIFY3U(sm_free_space, ==, msp_free_space);
}

/*
 * ==========================================================================
 * Metaslab groups
 * ==========================================================================
 */
/*
 * Update the allocatable flag and the metaslab group's capacity.
 * The allocatable flag is set to true if the capacity is below
 * the zfs_mg_noalloc_threshold or has a fragmentation value that is
 * greater than zfs_mg_fragmentation_threshold. If a metaslab group
 * transitions from allocatable to non-allocatable or vice versa then the
 * metaslab group's class is updated to reflect the transition.
 */
static void
metaslab_group_alloc_update(metaslab_group_t *mg)
{
        vdev_t *vd = mg->mg_vd;
        metaslab_class_t *mc = mg->mg_class;
        vdev_stat_t *vs = &vd->vdev_stat;
        boolean_t was_allocatable;
        boolean_t was_initialized;

        ASSERT(vd == vd->vdev_top);
        ASSERT3U(spa_config_held(mc->mc_spa, SCL_ALLOC, RW_READER), ==,
            SCL_ALLOC);

        mutex_enter(&mg->mg_lock);
        was_allocatable = mg->mg_allocatable;
        was_initialized = mg->mg_initialized;

        mg->mg_free_capacity = ((vs->vs_space - vs->vs_alloc) * 100) /
            (vs->vs_space + 1);

        mutex_enter(&mc->mc_lock);

        /*
         * If the metaslab group was just added then it won't
         * have any space until we finish syncing out this txg.
         * At that point we will consider it initialized and available
         * for allocations.  We also don't consider non-activated
         * metaslab groups (e.g. vdevs that are in the middle of being removed)
         * to be initialized, because they can't be used for allocation.
         */
        mg->mg_initialized = metaslab_group_initialized(mg);
        if (!was_initialized && mg->mg_initialized) {
                mc->mc_groups++;
        } else if (was_initialized && !mg->mg_initialized) {
                ASSERT3U(mc->mc_groups, >, 0);
                mc->mc_groups--;
        }
        if (mg->mg_initialized)
                mg->mg_no_free_space = B_FALSE;

        /*
         * A metaslab group is considered allocatable if it has plenty
         * of free space or is not heavily fragmented. We only take
         * fragmentation into account if the metaslab group has a valid
         * fragmentation metric (i.e. a value between 0 and 100).
         */
        mg->mg_allocatable = (mg->mg_activation_count > 0 &&
            mg->mg_free_capacity > zfs_mg_noalloc_threshold &&
            (mg->mg_fragmentation == ZFS_FRAG_INVALID ||
            mg->mg_fragmentation <= zfs_mg_fragmentation_threshold));

        /*
         * The mc_alloc_groups maintains a count of the number of
         * groups in this metaslab class that are still above the
         * zfs_mg_noalloc_threshold. This is used by the allocating
         * threads to determine if they should avoid allocations to
         * a given group. The allocator will avoid allocations to a group
         * if that group has reached or is below the zfs_mg_noalloc_threshold
         * and there are still other groups that are above the threshold.
         * When a group transitions from allocatable to non-allocatable or
         * vice versa we update the metaslab class to reflect that change.
         * When the mc_alloc_groups value drops to 0 that means that all
         * groups have reached the zfs_mg_noalloc_threshold making all groups
         * eligible for allocations. This effectively means that all devices
         * are balanced again.
         */
        if (was_allocatable && !mg->mg_allocatable)
                mc->mc_alloc_groups--;
        else if (!was_allocatable && mg->mg_allocatable)
                mc->mc_alloc_groups++;
        mutex_exit(&mc->mc_lock);

        mutex_exit(&mg->mg_lock);
}

metaslab_group_t *
metaslab_group_create(metaslab_class_t *mc, vdev_t *vd)
{
        metaslab_group_t *mg;

        mg = kmem_zalloc(sizeof (metaslab_group_t), KM_SLEEP);
        mutex_init(&mg->mg_lock, NULL, MUTEX_DEFAULT, NULL);
        avl_create(&mg->mg_metaslab_tree, metaslab_compare,
            sizeof (metaslab_t), offsetof(struct metaslab, ms_group_node));
        mg->mg_vd = vd;
        mg->mg_class = mc;
        mg->mg_activation_count = 0;
        mg->mg_initialized = B_FALSE;
        mg->mg_no_free_space = B_TRUE;
        refcount_create_tracked(&mg->mg_alloc_queue_depth);

        mg->mg_taskq = taskq_create("metaslab_group_taskq", metaslab_load_pct,
            minclsyspri, 10, INT_MAX, TASKQ_THREADS_CPU_PCT);

        return (mg);
}

void
metaslab_group_destroy(metaslab_group_t *mg)
{
        ASSERT(mg->mg_prev == NULL);
        ASSERT(mg->mg_next == NULL);
        /*
         * We may have gone below zero with the activation count
         * either because we never activated in the first place or
         * because we're done, and possibly removing the vdev.
         */
        ASSERT(mg->mg_activation_count <= 0);

        taskq_destroy(mg->mg_taskq);
        avl_destroy(&mg->mg_metaslab_tree);
        mutex_destroy(&mg->mg_lock);
        refcount_destroy(&mg->mg_alloc_queue_depth);
        kmem_free(mg, sizeof (metaslab_group_t));
}

void
metaslab_group_activate(metaslab_group_t *mg)
{
        metaslab_class_t *mc = mg->mg_class;
        metaslab_group_t *mgprev, *mgnext;

        ASSERT3U(spa_config_held(mc->mc_spa, SCL_ALLOC, RW_WRITER), !=, 0);

        ASSERT(mc->mc_rotor != mg);
        ASSERT(mg->mg_prev == NULL);
        ASSERT(mg->mg_next == NULL);
        ASSERT(mg->mg_activation_count <= 0);

        if (++mg->mg_activation_count <= 0)
                return;

        mg->mg_aliquot = metaslab_aliquot * MAX(1, mg->mg_vd->vdev_children);
        metaslab_group_alloc_update(mg);

        if ((mgprev = mc->mc_rotor) == NULL) {
                mg->mg_prev = mg;
                mg->mg_next = mg;
        } else {
                mgnext = mgprev->mg_next;
                mg->mg_prev = mgprev;
                mg->mg_next = mgnext;
                mgprev->mg_next = mg;
                mgnext->mg_prev = mg;
        }
        mc->mc_rotor = mg;
}

/*
 * Passivate a metaslab group and remove it from the allocation rotor.
 * Callers must hold both the SCL_ALLOC and SCL_ZIO lock prior to passivating
 * a metaslab group. This function will momentarily drop spa_config_locks
 * that are lower than the SCL_ALLOC lock (see comment below).
 */
void
metaslab_group_passivate(metaslab_group_t *mg)
{
        metaslab_class_t *mc = mg->mg_class;
        spa_t *spa = mc->mc_spa;
        metaslab_group_t *mgprev, *mgnext;
        int locks = spa_config_held(spa, SCL_ALL, RW_WRITER);

        ASSERT3U(spa_config_held(spa, SCL_ALLOC | SCL_ZIO, RW_WRITER), ==,
            (SCL_ALLOC | SCL_ZIO));

        if (--mg->mg_activation_count != 0) {
                ASSERT(mc->mc_rotor != mg);
                ASSERT(mg->mg_prev == NULL);
                ASSERT(mg->mg_next == NULL);
                ASSERT(mg->mg_activation_count < 0);
                return;
        }

        /*
         * The spa_config_lock is an array of rwlocks, ordered as
         * follows (from highest to lowest):
         *      SCL_CONFIG > SCL_STATE > SCL_L2ARC > SCL_ALLOC >
         *      SCL_ZIO > SCL_FREE > SCL_VDEV
         * (For more information about the spa_config_lock see spa_misc.c)
         * The higher the lock, the broader its coverage. When we passivate
         * a metaslab group, we must hold both the SCL_ALLOC and the SCL_ZIO
         * config locks. However, the metaslab group's taskq might be trying
         * to preload metaslabs so we must drop the SCL_ZIO lock and any
         * lower locks to allow the I/O to complete. At a minimum,
         * we continue to hold the SCL_ALLOC lock, which prevents any future
         * allocations from taking place and any changes to the vdev tree.
         */
        spa_config_exit(spa, locks & ~(SCL_ZIO - 1), spa);
        taskq_wait(mg->mg_taskq);
        spa_config_enter(spa, locks & ~(SCL_ZIO - 1), spa, RW_WRITER);
        metaslab_group_alloc_update(mg);

        mgprev = mg->mg_prev;
        mgnext = mg->mg_next;

        if (mg == mgnext) {
                mc->mc_rotor = NULL;
        } else {
                mc->mc_rotor = mgnext;
                mgprev->mg_next = mgnext;
                mgnext->mg_prev = mgprev;
        }

        mg->mg_prev = NULL;
        mg->mg_next = NULL;
}

boolean_t
metaslab_group_initialized(metaslab_group_t *mg)
{
        vdev_t *vd = mg->mg_vd;
        vdev_stat_t *vs = &vd->vdev_stat;

        return (vs->vs_space != 0 && mg->mg_activation_count > 0);
}

uint64_t
metaslab_group_get_space(metaslab_group_t *mg)
{
        return ((1ULL << mg->mg_vd->vdev_ms_shift) * mg->mg_vd->vdev_ms_count);
}

void
metaslab_group_histogram_verify(metaslab_group_t *mg)
{
        uint64_t *mg_hist;
        vdev_t *vd = mg->mg_vd;
        uint64_t ashift = vd->vdev_ashift;
        int i;

        if ((zfs_flags & ZFS_DEBUG_HISTOGRAM_VERIFY) == 0)
                return;

        mg_hist = kmem_zalloc(sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE,
            KM_SLEEP);

        ASSERT3U(RANGE_TREE_HISTOGRAM_SIZE, >=,
            SPACE_MAP_HISTOGRAM_SIZE + ashift);

        for (int m = 0; m < vd->vdev_ms_count; m++) {
                metaslab_t *msp = vd->vdev_ms[m];

                if (msp->ms_sm == NULL)
                        continue;

                for (i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++)
                        mg_hist[i + ashift] +=
                            msp->ms_sm->sm_phys->smp_histogram[i];
        }

        for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i ++)
                VERIFY3U(mg_hist[i], ==, mg->mg_histogram[i]);

        kmem_free(mg_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
}

static void
metaslab_group_histogram_add(metaslab_group_t *mg, metaslab_t *msp)
{
        metaslab_class_t *mc = mg->mg_class;
        uint64_t ashift = mg->mg_vd->vdev_ashift;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        if (msp->ms_sm == NULL)
                return;

        mutex_enter(&mg->mg_lock);
        for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
                mg->mg_histogram[i + ashift] +=
                    msp->ms_sm->sm_phys->smp_histogram[i];
                mc->mc_histogram[i + ashift] +=
                    msp->ms_sm->sm_phys->smp_histogram[i];
        }
        mutex_exit(&mg->mg_lock);
}

void
metaslab_group_histogram_remove(metaslab_group_t *mg, metaslab_t *msp)
{
        metaslab_class_t *mc = mg->mg_class;
        uint64_t ashift = mg->mg_vd->vdev_ashift;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        if (msp->ms_sm == NULL)
                return;

        mutex_enter(&mg->mg_lock);
        for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
                ASSERT3U(mg->mg_histogram[i + ashift], >=,
                    msp->ms_sm->sm_phys->smp_histogram[i]);
                ASSERT3U(mc->mc_histogram[i + ashift], >=,
                    msp->ms_sm->sm_phys->smp_histogram[i]);

                mg->mg_histogram[i + ashift] -=
                    msp->ms_sm->sm_phys->smp_histogram[i];
                mc->mc_histogram[i + ashift] -=
                    msp->ms_sm->sm_phys->smp_histogram[i];
        }
        mutex_exit(&mg->mg_lock);
}

static void
metaslab_group_add(metaslab_group_t *mg, metaslab_t *msp)
{
        ASSERT(msp->ms_group == NULL);
        mutex_enter(&mg->mg_lock);
        msp->ms_group = mg;
        msp->ms_weight = 0;
        avl_add(&mg->mg_metaslab_tree, msp);
        mutex_exit(&mg->mg_lock);

        mutex_enter(&msp->ms_lock);
        metaslab_group_histogram_add(mg, msp);
        mutex_exit(&msp->ms_lock);
}

static void
metaslab_group_remove(metaslab_group_t *mg, metaslab_t *msp)
{
        mutex_enter(&msp->ms_lock);
        metaslab_group_histogram_remove(mg, msp);
        mutex_exit(&msp->ms_lock);

        mutex_enter(&mg->mg_lock);
        ASSERT(msp->ms_group == mg);
        avl_remove(&mg->mg_metaslab_tree, msp);
        msp->ms_group = NULL;
        mutex_exit(&mg->mg_lock);
}

static void
metaslab_group_sort(metaslab_group_t *mg, metaslab_t *msp, uint64_t weight)
{
        /*
         * Although in principle the weight can be any value, in
         * practice we do not use values in the range [1, 511].
         */
        ASSERT(weight >= SPA_MINBLOCKSIZE || weight == 0);
        ASSERT(MUTEX_HELD(&msp->ms_lock));

        mutex_enter(&mg->mg_lock);
        ASSERT(msp->ms_group == mg);
        avl_remove(&mg->mg_metaslab_tree, msp);
        msp->ms_weight = weight;
        avl_add(&mg->mg_metaslab_tree, msp);
        mutex_exit(&mg->mg_lock);
}

/*
 * Calculate the fragmentation for a given metaslab group. We can use
 * a simple average here since all metaslabs within the group must have
 * the same size. The return value will be a value between 0 and 100
 * (inclusive), or ZFS_FRAG_INVALID if less than half of the metaslab in this
 * group have a fragmentation metric.
 */
uint64_t
metaslab_group_fragmentation(metaslab_group_t *mg)
{
        vdev_t *vd = mg->mg_vd;
        uint64_t fragmentation = 0;
        uint64_t valid_ms = 0;

        for (int m = 0; m < vd->vdev_ms_count; m++) {
                metaslab_t *msp = vd->vdev_ms[m];

                if (msp->ms_fragmentation == ZFS_FRAG_INVALID)
                        continue;

                valid_ms++;
                fragmentation += msp->ms_fragmentation;
        }

        if (valid_ms <= vd->vdev_ms_count / 2)
                return (ZFS_FRAG_INVALID);

        fragmentation /= valid_ms;
        ASSERT3U(fragmentation, <=, 100);
        return (fragmentation);
}

/*
 * Determine if a given metaslab group should skip allocations. A metaslab
 * group should avoid allocations if its free capacity is less than the
 * zfs_mg_noalloc_threshold or its fragmentation metric is greater than
 * zfs_mg_fragmentation_threshold and there is at least one metaslab group
 * that can still handle allocations. If the allocation throttle is enabled
 * then we skip allocations to devices that have reached their maximum
 * allocation queue depth unless the selected metaslab group is the only
 * eligible group remaining.
 */
static boolean_t
metaslab_group_allocatable(metaslab_group_t *mg, metaslab_group_t *rotor,
    uint64_t psize)
{
        spa_t *spa = mg->mg_vd->vdev_spa;
        metaslab_class_t *mc = mg->mg_class;

        /*
         * We can only consider skipping this metaslab group if it's
         * in the normal metaslab class and there are other metaslab
         * groups to select from. Otherwise, we always consider it eligible
         * for allocations.
         */
        if (mc != spa_normal_class(spa) || mc->mc_groups <= 1)
                return (B_TRUE);

        /*
         * If the metaslab group's mg_allocatable flag is set (see comments
         * in metaslab_group_alloc_update() for more information) and
         * the allocation throttle is disabled then allow allocations to this
         * device. However, if the allocation throttle is enabled then
         * check if we have reached our allocation limit (mg_alloc_queue_depth)
         * to determine if we should allow allocations to this metaslab group.
         * If all metaslab groups are no longer considered allocatable
         * (mc_alloc_groups == 0) or we're trying to allocate the smallest
         * gang block size then we allow allocations on this metaslab group
         * regardless of the mg_allocatable or throttle settings.
         */
        if (mg->mg_allocatable) {
                metaslab_group_t *mgp;
                int64_t qdepth;
                uint64_t qmax = mg->mg_max_alloc_queue_depth;

                if (!mc->mc_alloc_throttle_enabled)
                        return (B_TRUE);

                /*
                 * If this metaslab group does not have any free space, then
                 * there is no point in looking further.
                 */
                if (mg->mg_no_free_space)
                        return (B_FALSE);

                qdepth = refcount_count(&mg->mg_alloc_queue_depth);

                /*
                 * If this metaslab group is below its qmax or it's
                 * the only allocatable metasable group, then attempt
                 * to allocate from it.
                 */
                if (qdepth < qmax || mc->mc_alloc_groups == 1)
                        return (B_TRUE);
                ASSERT3U(mc->mc_alloc_groups, >, 1);

                /*
                 * Since this metaslab group is at or over its qmax, we
                 * need to determine if there are metaslab groups after this
                 * one that might be able to handle this allocation. This is
                 * racy since we can't hold the locks for all metaslab
                 * groups at the same time when we make this check.
                 */
                for (mgp = mg->mg_next; mgp != rotor; mgp = mgp->mg_next) {
                        qmax = mgp->mg_max_alloc_queue_depth;

                        qdepth = refcount_count(&mgp->mg_alloc_queue_depth);

                        /*
                         * If there is another metaslab group that
                         * might be able to handle the allocation, then
                         * we return false so that we skip this group.
                         */
                        if (qdepth < qmax && !mgp->mg_no_free_space)
                                return (B_FALSE);
                }

                /*
                 * We didn't find another group to handle the allocation
                 * so we can't skip this metaslab group even though
                 * we are at or over our qmax.
                 */
                return (B_TRUE);

        } else if (mc->mc_alloc_groups == 0 || psize == SPA_MINBLOCKSIZE) {
                return (B_TRUE);
        }
        return (B_FALSE);
}

/*
 * ==========================================================================
 * Range tree callbacks
 * ==========================================================================
 */

/*
 * Comparison function for the private size-ordered tree. Tree is sorted
 * by size, larger sizes at the end of the tree.
 */
static int
metaslab_rangesize_compare(const void *x1, const void *x2)
{
        const range_seg_t *r1 = x1;
        const range_seg_t *r2 = x2;
        uint64_t rs_size1 = r1->rs_end - r1->rs_start;
        uint64_t rs_size2 = r2->rs_end - r2->rs_start;

        if (rs_size1 < rs_size2)
                return (-1);
        if (rs_size1 > rs_size2)
                return (1);

        if (r1->rs_start < r2->rs_start)
                return (-1);

        if (r1->rs_start > r2->rs_start)
                return (1);

        return (0);
}

/*
 * Create any block allocator specific components. The current allocators
 * rely on using both a size-ordered range_tree_t and an array of uint64_t's.
 */
static void
metaslab_rt_create(range_tree_t *rt, void *arg)
{
        metaslab_t *msp = arg;

        ASSERT3P(rt->rt_arg, ==, msp);
        ASSERT(msp->ms_allocatable == NULL);

        avl_create(&msp->ms_allocatable_by_size, metaslab_rangesize_compare,
            sizeof (range_seg_t), offsetof(range_seg_t, rs_pp_node));
}

/*
 * Destroy the block allocator specific components.
 */
static void
metaslab_rt_destroy(range_tree_t *rt, void *arg)
{
        metaslab_t *msp = arg;

        ASSERT3P(rt->rt_arg, ==, msp);
        ASSERT3P(msp->ms_allocatable, ==, rt);
        ASSERT0(avl_numnodes(&msp->ms_allocatable_by_size));

        avl_destroy(&msp->ms_allocatable_by_size);
}

static void
metaslab_rt_add(range_tree_t *rt, range_seg_t *rs, void *arg)
{
        metaslab_t *msp = arg;

        ASSERT3P(rt->rt_arg, ==, msp);
        ASSERT3P(msp->ms_allocatable, ==, rt);
        VERIFY(!msp->ms_condensing);
        avl_add(&msp->ms_allocatable_by_size, rs);
}

static void
metaslab_rt_remove(range_tree_t *rt, range_seg_t *rs, void *arg)
{
        metaslab_t *msp = arg;

        ASSERT3P(rt->rt_arg, ==, msp);
        ASSERT3P(msp->ms_allocatable, ==, rt);
        VERIFY(!msp->ms_condensing);
        avl_remove(&msp->ms_allocatable_by_size, rs);
}

static void
metaslab_rt_vacate(range_tree_t *rt, void *arg)
{
        metaslab_t *msp = arg;

        ASSERT3P(rt->rt_arg, ==, msp);
        ASSERT3P(msp->ms_allocatable, ==, rt);

        /*
         * Normally one would walk the tree freeing nodes along the way.
         * Since the nodes are shared with the range trees we can avoid
         * walking all nodes and just reinitialize the avl tree. The nodes
         * will be freed by the range tree, so we don't want to free them here.
         */
        avl_create(&msp->ms_allocatable_by_size, metaslab_rangesize_compare,
            sizeof (range_seg_t), offsetof(range_seg_t, rs_pp_node));
}

static range_tree_ops_t metaslab_rt_ops = {
        metaslab_rt_create,
        metaslab_rt_destroy,
        metaslab_rt_add,
        metaslab_rt_remove,
        metaslab_rt_vacate
};

/*
 * ==========================================================================
 * Common allocator routines
 * ==========================================================================
 */

/*
 * Return the maximum contiguous segment within the metaslab.
 */
uint64_t
metaslab_block_maxsize(metaslab_t *msp)
{
        avl_tree_t *t = &msp->ms_allocatable_by_size;
        range_seg_t *rs;

        if (t == NULL || (rs = avl_last(t)) == NULL)
                return (0ULL);

        return (rs->rs_end - rs->rs_start);
}

static range_seg_t *
metaslab_block_find(avl_tree_t *t, uint64_t start, uint64_t size)
{
        range_seg_t *rs, rsearch;
        avl_index_t where;

        rsearch.rs_start = start;
        rsearch.rs_end = start + size;

        rs = avl_find(t, &rsearch, &where);
        if (rs == NULL) {
                rs = avl_nearest(t, where, AVL_AFTER);
        }

        return (rs);
}

/*
 * This is a helper function that can be used by the allocator to find
 * a suitable block to allocate. This will search the specified AVL
 * tree looking for a block that matches the specified criteria.
 */
static uint64_t
metaslab_block_picker(avl_tree_t *t, uint64_t *cursor, uint64_t size,
    uint64_t align)
{
        range_seg_t *rs = metaslab_block_find(t, *cursor, size);

        while (rs != NULL) {
                uint64_t offset = P2ROUNDUP(rs->rs_start, align);

                if (offset + size <= rs->rs_end) {
                        *cursor = offset + size;
                        return (offset);
                }
                rs = AVL_NEXT(t, rs);
        }

        /*
         * If we know we've searched the whole map (*cursor == 0), give up.
         * Otherwise, reset the cursor to the beginning and try again.
         */
        if (*cursor == 0)
                return (-1ULL);

        *cursor = 0;
        return (metaslab_block_picker(t, cursor, size, align));
}

/*
 * ==========================================================================
 * The first-fit block allocator
 * ==========================================================================
 */
static uint64_t
metaslab_ff_alloc(metaslab_t *msp, uint64_t size)
{
        /*
         * Find the largest power of 2 block size that evenly divides the
         * requested size. This is used to try to allocate blocks with similar
         * alignment from the same area of the metaslab (i.e. same cursor
         * bucket) but it does not guarantee that other allocations sizes
         * may exist in the same region.
         */
        uint64_t align = size & -size;
        uint64_t *cursor = &msp->ms_lbas[highbit64(align) - 1];
        avl_tree_t *t = &msp->ms_allocatable->rt_root;

        return (metaslab_block_picker(t, cursor, size, align));
}

static metaslab_ops_t metaslab_ff_ops = {
        metaslab_ff_alloc
};

/*
 * ==========================================================================
 * Dynamic block allocator -
 * Uses the first fit allocation scheme until space get low and then
 * adjusts to a best fit allocation method. Uses metaslab_df_alloc_threshold
 * and metaslab_df_free_pct to determine when to switch the allocation scheme.
 * ==========================================================================
 */
static uint64_t
metaslab_df_alloc(metaslab_t *msp, uint64_t size)
{
        /*
         * Find the largest power of 2 block size that evenly divides the
         * requested size. This is used to try to allocate blocks with similar
         * alignment from the same area of the metaslab (i.e. same cursor
         * bucket) but it does not guarantee that other allocations sizes
         * may exist in the same region.
         */
        uint64_t align = size & -size;
        uint64_t *cursor = &msp->ms_lbas[highbit64(align) - 1];
        range_tree_t *rt = msp->ms_allocatable;
        avl_tree_t *t = &rt->rt_root;
        uint64_t max_size = metaslab_block_maxsize(msp);
        int free_pct = range_tree_space(rt) * 100 / msp->ms_size;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT3U(avl_numnodes(t), ==,
            avl_numnodes(&msp->ms_allocatable_by_size));

        if (max_size < size)
                return (-1ULL);

        /*
         * If we're running low on space switch to using the size
         * sorted AVL tree (best-fit).
         */
        if (max_size < metaslab_df_alloc_threshold ||
            free_pct < metaslab_df_free_pct) {
                t = &msp->ms_allocatable_by_size;
                *cursor = 0;
        }

        return (metaslab_block_picker(t, cursor, size, 1ULL));
}

static metaslab_ops_t metaslab_df_ops = {
        metaslab_df_alloc
};

/*
 * ==========================================================================
 * Cursor fit block allocator -
 * Select the largest region in the metaslab, set the cursor to the beginning
 * of the range and the cursor_end to the end of the range. As allocations
 * are made advance the cursor. Continue allocating from the cursor until
 * the range is exhausted and then find a new range.
 * ==========================================================================
 */
static uint64_t
metaslab_cf_alloc(metaslab_t *msp, uint64_t size)
{
        range_tree_t *rt = msp->ms_allocatable;
        avl_tree_t *t = &msp->ms_allocatable_by_size;
        uint64_t *cursor = &msp->ms_lbas[0];
        uint64_t *cursor_end = &msp->ms_lbas[1];
        uint64_t offset = 0;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT3U(avl_numnodes(t), ==, avl_numnodes(&rt->rt_root));

        ASSERT3U(*cursor_end, >=, *cursor);

        if ((*cursor + size) > *cursor_end) {
                range_seg_t *rs;

                rs = avl_last(&msp->ms_allocatable_by_size);
                if (rs == NULL || (rs->rs_end - rs->rs_start) < size)
                        return (-1ULL);

                *cursor = rs->rs_start;
                *cursor_end = rs->rs_end;
        }

        offset = *cursor;
        *cursor += size;

        return (offset);
}

static metaslab_ops_t metaslab_cf_ops = {
        metaslab_cf_alloc
};

/*
 * ==========================================================================
 * New dynamic fit allocator -
 * Select a region that is large enough to allocate 2^metaslab_ndf_clump_shift
 * contiguous blocks. If no region is found then just use the largest segment
 * that remains.
 * ==========================================================================
 */

/*
 * Determines desired number of contiguous blocks (2^metaslab_ndf_clump_shift)
 * to request from the allocator.
 */
uint64_t metaslab_ndf_clump_shift = 4;

static uint64_t
metaslab_ndf_alloc(metaslab_t *msp, uint64_t size)
{
        avl_tree_t *t = &msp->ms_allocatable->rt_root;
        avl_index_t where;
        range_seg_t *rs, rsearch;
        uint64_t hbit = highbit64(size);
        uint64_t *cursor = &msp->ms_lbas[hbit - 1];
        uint64_t max_size = metaslab_block_maxsize(msp);

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT3U(avl_numnodes(t), ==,
            avl_numnodes(&msp->ms_allocatable_by_size));

        if (max_size < size)
                return (-1ULL);

        rsearch.rs_start = *cursor;
        rsearch.rs_end = *cursor + size;

        rs = avl_find(t, &rsearch, &where);
        if (rs == NULL || (rs->rs_end - rs->rs_start) < size) {
                t = &msp->ms_allocatable_by_size;

                rsearch.rs_start = 0;
                rsearch.rs_end = MIN(max_size,
                    1ULL << (hbit + metaslab_ndf_clump_shift));
                rs = avl_find(t, &rsearch, &where);
                if (rs == NULL)
                        rs = avl_nearest(t, where, AVL_AFTER);
                ASSERT(rs != NULL);
        }

        if ((rs->rs_end - rs->rs_start) >= size) {
                *cursor = rs->rs_start + size;
                return (rs->rs_start);
        }
        return (-1ULL);
}

static metaslab_ops_t metaslab_ndf_ops = {
        metaslab_ndf_alloc
};

metaslab_ops_t *zfs_metaslab_ops = &metaslab_df_ops;

/*
 * ==========================================================================
 * Metaslabs
 * ==========================================================================
 */

/*
 * Wait for any in-progress metaslab loads to complete.
 */
void
metaslab_load_wait(metaslab_t *msp)
{
        ASSERT(MUTEX_HELD(&msp->ms_lock));

        while (msp->ms_loading) {
                ASSERT(!msp->ms_loaded);
                cv_wait(&msp->ms_load_cv, &msp->ms_lock);
        }
}

int
metaslab_load(metaslab_t *msp)
{
        int error = 0;
        boolean_t success = B_FALSE;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT(!msp->ms_loaded);
        ASSERT(!msp->ms_loading);

        msp->ms_loading = B_TRUE;
        /*
         * Nobody else can manipulate a loading metaslab, so it's now safe
         * to drop the lock.  This way we don't have to hold the lock while
         * reading the spacemap from disk.
         */
        mutex_exit(&msp->ms_lock);

        /*
         * If the space map has not been allocated yet, then treat
         * all the space in the metaslab as free and add it to ms_allocatable.
         */
        if (msp->ms_sm != NULL) {
                error = space_map_load(msp->ms_sm, msp->ms_allocatable,
                    SM_FREE);
        } else {
                range_tree_add(msp->ms_allocatable,
                    msp->ms_start, msp->ms_size);
        }

        success = (error == 0);

        mutex_enter(&msp->ms_lock);
        msp->ms_loading = B_FALSE;

        if (success) {
                ASSERT3P(msp->ms_group, !=, NULL);
                msp->ms_loaded = B_TRUE;

                /*
                 * If the metaslab already has a spacemap, then we need to
                 * remove all segments from the defer tree; otherwise, the
                 * metaslab is completely empty and we can skip this.
                 */
                if (msp->ms_sm != NULL) {
                        for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                                range_tree_walk(msp->ms_defer[t],
                                    range_tree_remove, msp->ms_allocatable);
                        }
                }
                msp->ms_max_size = metaslab_block_maxsize(msp);
        }
        cv_broadcast(&msp->ms_load_cv);
        return (error);
}

void
metaslab_unload(metaslab_t *msp)
{
        ASSERT(MUTEX_HELD(&msp->ms_lock));
        range_tree_vacate(msp->ms_allocatable, NULL, NULL);
        msp->ms_loaded = B_FALSE;
        msp->ms_weight &= ~METASLAB_ACTIVE_MASK;
        msp->ms_max_size = 0;
}

int
metaslab_init(metaslab_group_t *mg, uint64_t id, uint64_t object, uint64_t txg,
    metaslab_t **msp)
{
        vdev_t *vd = mg->mg_vd;
        objset_t *mos = vd->vdev_spa->spa_meta_objset;
        metaslab_t *ms;
        int error;

        ms = kmem_zalloc(sizeof (metaslab_t), KM_SLEEP);
        mutex_init(&ms->ms_lock, NULL, MUTEX_DEFAULT, NULL);
        mutex_init(&ms->ms_sync_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&ms->ms_load_cv, NULL, CV_DEFAULT, NULL);
        ms->ms_id = id;
        ms->ms_start = id << vd->vdev_ms_shift;
        ms->ms_size = 1ULL << vd->vdev_ms_shift;

        /*
         * We only open space map objects that already exist. All others
         * will be opened when we finally allocate an object for it.
         */
        if (object != 0) {
                error = space_map_open(&ms->ms_sm, mos, object, ms->ms_start,
                    ms->ms_size, vd->vdev_ashift);

                if (error != 0) {
                        kmem_free(ms, sizeof (metaslab_t));
                        return (error);
                }

                ASSERT(ms->ms_sm != NULL);
        }

        /*
         * We create the main range tree here, but we don't create the
         * other range trees until metaslab_sync_done().  This serves
         * two purposes: it allows metaslab_sync_done() to detect the
         * addition of new space; and for debugging, it ensures that we'd
         * data fault on any attempt to use this metaslab before it's ready.
         */
        ms->ms_allocatable = range_tree_create(&metaslab_rt_ops, ms);
        metaslab_group_add(mg, ms);

        metaslab_set_fragmentation(ms);

        /*
         * If we're opening an existing pool (txg == 0) or creating
         * a new one (txg == TXG_INITIAL), all space is available now.
         * If we're adding space to an existing pool, the new space
         * does not become available until after this txg has synced.
         * The metaslab's weight will also be initialized when we sync
         * out this txg. This ensures that we don't attempt to allocate
         * from it before we have initialized it completely.
         */
        if (txg <= TXG_INITIAL)
                metaslab_sync_done(ms, 0);

        /*
         * If metaslab_debug_load is set and we're initializing a metaslab
         * that has an allocated space map object then load the its space
         * map so that can verify frees.
         */
        if (metaslab_debug_load && ms->ms_sm != NULL) {
                mutex_enter(&ms->ms_lock);
                VERIFY0(metaslab_load(ms));
                mutex_exit(&ms->ms_lock);
        }

        if (txg != 0) {
                vdev_dirty(vd, 0, NULL, txg);
                vdev_dirty(vd, VDD_METASLAB, ms, txg);
        }

        *msp = ms;

        return (0);
}

void
metaslab_fini(metaslab_t *msp)
{
        metaslab_group_t *mg = msp->ms_group;

        metaslab_group_remove(mg, msp);

        mutex_enter(&msp->ms_lock);
        VERIFY(msp->ms_group == NULL);
        vdev_space_update(mg->mg_vd, -space_map_allocated(msp->ms_sm),
            0, -msp->ms_size);
        space_map_close(msp->ms_sm);

        metaslab_unload(msp);
        range_tree_destroy(msp->ms_allocatable);
        range_tree_destroy(msp->ms_freeing);
        range_tree_destroy(msp->ms_freed);

        for (int t = 0; t < TXG_SIZE; t++) {
                range_tree_destroy(msp->ms_allocating[t]);
        }

        for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                range_tree_destroy(msp->ms_defer[t]);
        }
        ASSERT0(msp->ms_deferspace);

        range_tree_destroy(msp->ms_checkpointing);

        mutex_exit(&msp->ms_lock);
        cv_destroy(&msp->ms_load_cv);
        mutex_destroy(&msp->ms_lock);
        mutex_destroy(&msp->ms_sync_lock);

        kmem_free(msp, sizeof (metaslab_t));
}

#define FRAGMENTATION_TABLE_SIZE        17

/*
 * This table defines a segment size based fragmentation metric that will
 * allow each metaslab to derive its own fragmentation value. This is done
 * by calculating the space in each bucket of the spacemap histogram and
 * multiplying that by the fragmetation metric in this table. Doing
 * this for all buckets and dividing it by the total amount of free
 * space in this metaslab (i.e. the total free space in all buckets) gives
 * us the fragmentation metric. This means that a high fragmentation metric
 * equates to most of the free space being comprised of small segments.
 * Conversely, if the metric is low, then most of the free space is in
 * large segments. A 10% change in fragmentation equates to approximately
 * double the number of segments.
 *
 * This table defines 0% fragmented space using 16MB segments. Testing has
 * shown that segments that are greater than or equal to 16MB do not suffer
 * from drastic performance problems. Using this value, we derive the rest
 * of the table. Since the fragmentation value is never stored on disk, it
 * is possible to change these calculations in the future.
 */
int zfs_frag_table[FRAGMENTATION_TABLE_SIZE] = {
        100,    /* 512B */
        100,    /* 1K   */
        98,     /* 2K   */
        95,     /* 4K   */
        90,     /* 8K   */
        80,     /* 16K  */
        70,     /* 32K  */
        60,     /* 64K  */
        50,     /* 128K */
        40,     /* 256K */
        30,     /* 512K */
        20,     /* 1M   */
        15,     /* 2M   */
        10,     /* 4M   */
        5,      /* 8M   */
        0       /* 16M  */
};

/*
 * Calclate the metaslab's fragmentation metric. A return value
 * of ZFS_FRAG_INVALID means that the metaslab has not been upgraded and does
 * not support this metric. Otherwise, the return value should be in the
 * range [0, 100].
 */
static void
metaslab_set_fragmentation(metaslab_t *msp)
{
        spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
        uint64_t fragmentation = 0;
        uint64_t total = 0;
        boolean_t feature_enabled = spa_feature_is_enabled(spa,
            SPA_FEATURE_SPACEMAP_HISTOGRAM);

        if (!feature_enabled) {
                msp->ms_fragmentation = ZFS_FRAG_INVALID;
                return;
        }

        /*
         * A null space map means that the entire metaslab is free
         * and thus is not fragmented.
         */
        if (msp->ms_sm == NULL) {
                msp->ms_fragmentation = 0;
                return;
        }

        /*
         * If this metaslab's space map has not been upgraded, flag it
         * so that we upgrade next time we encounter it.
         */
        if (msp->ms_sm->sm_dbuf->db_size != sizeof (space_map_phys_t)) {
                uint64_t txg = spa_syncing_txg(spa);
                vdev_t *vd = msp->ms_group->mg_vd;

                /*
                 * If we've reached the final dirty txg, then we must
                 * be shutting down the pool. We don't want to dirty
                 * any data past this point so skip setting the condense
                 * flag. We can retry this action the next time the pool
                 * is imported.
                 */
                if (spa_writeable(spa) && txg < spa_final_dirty_txg(spa)) {
                        msp->ms_condense_wanted = B_TRUE;
                        vdev_dirty(vd, VDD_METASLAB, msp, txg + 1);
                        spa_dbgmsg(spa, "txg %llu, requesting force condense: "
                            "ms_id %llu, vdev_id %llu", txg, msp->ms_id,
                            vd->vdev_id);
                }
                msp->ms_fragmentation = ZFS_FRAG_INVALID;
                return;
        }

        for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
                uint64_t space = 0;
                uint8_t shift = msp->ms_sm->sm_shift;

                int idx = MIN(shift - SPA_MINBLOCKSHIFT + i,
                    FRAGMENTATION_TABLE_SIZE - 1);

                if (msp->ms_sm->sm_phys->smp_histogram[i] == 0)
                        continue;

                space = msp->ms_sm->sm_phys->smp_histogram[i] << (i + shift);
                total += space;

                ASSERT3U(idx, <, FRAGMENTATION_TABLE_SIZE);
                fragmentation += space * zfs_frag_table[idx];
        }

        if (total > 0)
                fragmentation /= total;
        ASSERT3U(fragmentation, <=, 100);

        msp->ms_fragmentation = fragmentation;
}

/*
 * Compute a weight -- a selection preference value -- for the given metaslab.
 * This is based on the amount of free space, the level of fragmentation,
 * the LBA range, and whether the metaslab is loaded.
 */
static uint64_t
metaslab_space_weight(metaslab_t *msp)
{
        metaslab_group_t *mg = msp->ms_group;
        vdev_t *vd = mg->mg_vd;
        uint64_t weight, space;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT(!vd->vdev_removing);

        /*
         * The baseline weight is the metaslab's free space.
         */
        space = msp->ms_size - space_map_allocated(msp->ms_sm);

        if (metaslab_fragmentation_factor_enabled &&
            msp->ms_fragmentation != ZFS_FRAG_INVALID) {
                /*
                 * Use the fragmentation information to inversely scale
                 * down the baseline weight. We need to ensure that we
                 * don't exclude this metaslab completely when it's 100%
                 * fragmented. To avoid this we reduce the fragmented value
                 * by 1.
                 */
                space = (space * (100 - (msp->ms_fragmentation - 1))) / 100;

                /*
                 * If space < SPA_MINBLOCKSIZE, then we will not allocate from
                 * this metaslab again. The fragmentation metric may have
                 * decreased the space to something smaller than
                 * SPA_MINBLOCKSIZE, so reset the space to SPA_MINBLOCKSIZE
                 * so that we can consume any remaining space.
                 */
                if (space > 0 && space < SPA_MINBLOCKSIZE)
                        space = SPA_MINBLOCKSIZE;
        }
        weight = space;

        /*
         * Modern disks have uniform bit density and constant angular velocity.
         * Therefore, the outer recording zones are faster (higher bandwidth)
         * than the inner zones by the ratio of outer to inner track diameter,
         * which is typically around 2:1.  We account for this by assigning
         * higher weight to lower metaslabs (multiplier ranging from 2x to 1x).
         * In effect, this means that we'll select the metaslab with the most
         * free bandwidth rather than simply the one with the most free space.
         */
        if (metaslab_lba_weighting_enabled) {
                weight = 2 * weight - (msp->ms_id * weight) / vd->vdev_ms_count;
                ASSERT(weight >= space && weight <= 2 * space);
        }

        /*
         * If this metaslab is one we're actively using, adjust its
         * weight to make it preferable to any inactive metaslab so
         * we'll polish it off. If the fragmentation on this metaslab
         * has exceed our threshold, then don't mark it active.
         */
        if (msp->ms_loaded && msp->ms_fragmentation != ZFS_FRAG_INVALID &&
            msp->ms_fragmentation <= zfs_metaslab_fragmentation_threshold) {
                weight |= (msp->ms_weight & METASLAB_ACTIVE_MASK);
        }

        WEIGHT_SET_SPACEBASED(weight);
        return (weight);
}

/*
 * Return the weight of the specified metaslab, according to the segment-based
 * weighting algorithm. The metaslab must be loaded. This function can
 * be called within a sync pass since it relies only on the metaslab's
 * range tree which is always accurate when the metaslab is loaded.
 */
static uint64_t
metaslab_weight_from_range_tree(metaslab_t *msp)
{
        uint64_t weight = 0;
        uint32_t segments = 0;

        ASSERT(msp->ms_loaded);

        for (int i = RANGE_TREE_HISTOGRAM_SIZE - 1; i >= SPA_MINBLOCKSHIFT;
            i--) {
                uint8_t shift = msp->ms_group->mg_vd->vdev_ashift;
                int max_idx = SPACE_MAP_HISTOGRAM_SIZE + shift - 1;

                segments <<= 1;
                segments += msp->ms_allocatable->rt_histogram[i];

                /*
                 * The range tree provides more precision than the space map
                 * and must be downgraded so that all values fit within the
                 * space map's histogram. This allows us to compare loaded
                 * vs. unloaded metaslabs to determine which metaslab is
                 * considered "best".
                 */
                if (i > max_idx)
                        continue;

                if (segments != 0) {
                        WEIGHT_SET_COUNT(weight, segments);
                        WEIGHT_SET_INDEX(weight, i);
                        WEIGHT_SET_ACTIVE(weight, 0);
                        break;
                }
        }
        return (weight);
}

/*
 * Calculate the weight based on the on-disk histogram. This should only
 * be called after a sync pass has completely finished since the on-disk
 * information is updated in metaslab_sync().
 */
static uint64_t
metaslab_weight_from_spacemap(metaslab_t *msp)
{
        uint64_t weight = 0;

        for (int i = SPACE_MAP_HISTOGRAM_SIZE - 1; i >= 0; i--) {
                if (msp->ms_sm->sm_phys->smp_histogram[i] != 0) {
                        WEIGHT_SET_COUNT(weight,
                            msp->ms_sm->sm_phys->smp_histogram[i]);
                        WEIGHT_SET_INDEX(weight, i +
                            msp->ms_sm->sm_shift);
                        WEIGHT_SET_ACTIVE(weight, 0);
                        break;
                }
        }
        return (weight);
}

/*
 * Compute a segment-based weight for the specified metaslab. The weight
 * is determined by highest bucket in the histogram. The information
 * for the highest bucket is encoded into the weight value.
 */
static uint64_t
metaslab_segment_weight(metaslab_t *msp)
{
        metaslab_group_t *mg = msp->ms_group;
        uint64_t weight = 0;
        uint8_t shift = mg->mg_vd->vdev_ashift;

        ASSERT(MUTEX_HELD(&msp->ms_lock));

        /*
         * The metaslab is completely free.
         */
        if (space_map_allocated(msp->ms_sm) == 0) {
                int idx = highbit64(msp->ms_size) - 1;
                int max_idx = SPACE_MAP_HISTOGRAM_SIZE + shift - 1;

                if (idx < max_idx) {
                        WEIGHT_SET_COUNT(weight, 1ULL);
                        WEIGHT_SET_INDEX(weight, idx);
                } else {
                        WEIGHT_SET_COUNT(weight, 1ULL << (idx - max_idx));
                        WEIGHT_SET_INDEX(weight, max_idx);
                }
                WEIGHT_SET_ACTIVE(weight, 0);
                ASSERT(!WEIGHT_IS_SPACEBASED(weight));

                return (weight);
        }

        ASSERT3U(msp->ms_sm->sm_dbuf->db_size, ==, sizeof (space_map_phys_t));

        /*
         * If the metaslab is fully allocated then just make the weight 0.
         */
        if (space_map_allocated(msp->ms_sm) == msp->ms_size)
                return (0);
        /*
         * If the metaslab is already loaded, then use the range tree to
         * determine the weight. Otherwise, we rely on the space map information
         * to generate the weight.
         */
        if (msp->ms_loaded) {
                weight = metaslab_weight_from_range_tree(msp);
        } else {
                weight = metaslab_weight_from_spacemap(msp);
        }

        /*
         * If the metaslab was active the last time we calculated its weight
         * then keep it active. We want to consume the entire region that
         * is associated with this weight.
         */
        if (msp->ms_activation_weight != 0 && weight != 0)
                WEIGHT_SET_ACTIVE(weight, WEIGHT_GET_ACTIVE(msp->ms_weight));
        return (weight);
}

/*
 * Determine if we should attempt to allocate from this metaslab. If the
 * metaslab has a maximum size then we can quickly determine if the desired
 * allocation size can be satisfied. Otherwise, if we're using segment-based
 * weighting then we can determine the maximum allocation that this metaslab
 * can accommodate based on the index encoded in the weight. If we're using
 * space-based weights then rely on the entire weight (excluding the weight
 * type bit).
 */
boolean_t
metaslab_should_allocate(metaslab_t *msp, uint64_t asize)
{
        boolean_t should_allocate;

        if (msp->ms_max_size != 0)
                return (msp->ms_max_size >= asize);

        if (!WEIGHT_IS_SPACEBASED(msp->ms_weight)) {
                /*
                 * The metaslab segment weight indicates segments in the
                 * range [2^i, 2^(i+1)), where i is the index in the weight.
                 * Since the asize might be in the middle of the range, we
                 * should attempt the allocation if asize < 2^(i+1).
                 */
                should_allocate = (asize <
                    1ULL << (WEIGHT_GET_INDEX(msp->ms_weight) + 1));
        } else {
                should_allocate = (asize <=
                    (msp->ms_weight & ~METASLAB_WEIGHT_TYPE));
        }
        return (should_allocate);
}

static uint64_t
metaslab_weight(metaslab_t *msp)
{
        vdev_t *vd = msp->ms_group->mg_vd;
        spa_t *spa = vd->vdev_spa;
        uint64_t weight;

        ASSERT(MUTEX_HELD(&msp->ms_lock));

        /*
         * If this vdev is in the process of being removed, there is nothing
         * for us to do here.
         */
        if (vd->vdev_removing)
                return (0);

        metaslab_set_fragmentation(msp);

        /*
         * Update the maximum size if the metaslab is loaded. This will
         * ensure that we get an accurate maximum size if newly freed space
         * has been added back into the free tree.
         */
        if (msp->ms_loaded)
                msp->ms_max_size = metaslab_block_maxsize(msp);

        /*
         * Segment-based weighting requires space map histogram support.
         */
        if (zfs_metaslab_segment_weight_enabled &&
            spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_HISTOGRAM) &&
            (msp->ms_sm == NULL || msp->ms_sm->sm_dbuf->db_size ==
            sizeof (space_map_phys_t))) {
                weight = metaslab_segment_weight(msp);
        } else {
                weight = metaslab_space_weight(msp);
        }
        return (weight);
}

static int
metaslab_activate(metaslab_t *msp, uint64_t activation_weight)
{
        ASSERT(MUTEX_HELD(&msp->ms_lock));

        if ((msp->ms_weight & METASLAB_ACTIVE_MASK) == 0) {
                metaslab_load_wait(msp);
                if (!msp->ms_loaded) {
                        int error = metaslab_load(msp);
                        if (error) {
                                metaslab_group_sort(msp->ms_group, msp, 0);
                                return (error);
                        }
                }

                msp->ms_activation_weight = msp->ms_weight;
                metaslab_group_sort(msp->ms_group, msp,
                    msp->ms_weight | activation_weight);
        }
        ASSERT(msp->ms_loaded);
        ASSERT(msp->ms_weight & METASLAB_ACTIVE_MASK);

        return (0);
}

static void
metaslab_passivate(metaslab_t *msp, uint64_t weight)
{
        uint64_t size = weight & ~METASLAB_WEIGHT_TYPE;

        /*
         * If size < SPA_MINBLOCKSIZE, then we will not allocate from
         * this metaslab again.  In that case, it had better be empty,
         * or we would be leaving space on the table.
         */
        ASSERT(size >= SPA_MINBLOCKSIZE ||
            range_tree_is_empty(msp->ms_allocatable));
        ASSERT0(weight & METASLAB_ACTIVE_MASK);

        msp->ms_activation_weight = 0;
        metaslab_group_sort(msp->ms_group, msp, weight);
        ASSERT((msp->ms_weight & METASLAB_ACTIVE_MASK) == 0);
}

/*
 * Segment-based metaslabs are activated once and remain active until
 * we either fail an allocation attempt (similar to space-based metaslabs)
 * or have exhausted the free space in zfs_metaslab_switch_threshold
 * buckets since the metaslab was activated. This function checks to see
 * if we've exhaused the zfs_metaslab_switch_threshold buckets in the
 * metaslab and passivates it proactively. This will allow us to select a
 * metaslabs with larger contiguous region if any remaining within this
 * metaslab group. If we're in sync pass > 1, then we continue using this
 * metaslab so that we don't dirty more block and cause more sync passes.
 */
void
metaslab_segment_may_passivate(metaslab_t *msp)
{
        spa_t *spa = msp->ms_group->mg_vd->vdev_spa;

        if (WEIGHT_IS_SPACEBASED(msp->ms_weight) || spa_sync_pass(spa) > 1)
                return;

        /*
         * Since we are in the middle of a sync pass, the most accurate
         * information that is accessible to us is the in-core range tree
         * histogram; calculate the new weight based on that information.
         */
        uint64_t weight = metaslab_weight_from_range_tree(msp);
        int activation_idx = WEIGHT_GET_INDEX(msp->ms_activation_weight);
        int current_idx = WEIGHT_GET_INDEX(weight);

        if (current_idx <= activation_idx - zfs_metaslab_switch_threshold)
                metaslab_passivate(msp, weight);
}

static void
metaslab_preload(void *arg)
{
        metaslab_t *msp = arg;
        spa_t *spa = msp->ms_group->mg_vd->vdev_spa;

        ASSERT(!MUTEX_HELD(&msp->ms_group->mg_lock));

        mutex_enter(&msp->ms_lock);
        metaslab_load_wait(msp);
        if (!msp->ms_loaded)
                (void) metaslab_load(msp);
        msp->ms_selected_txg = spa_syncing_txg(spa);
        mutex_exit(&msp->ms_lock);
}

static void
metaslab_group_preload(metaslab_group_t *mg)
{
        spa_t *spa = mg->mg_vd->vdev_spa;
        metaslab_t *msp;
        avl_tree_t *t = &mg->mg_metaslab_tree;
        int m = 0;

        if (spa_shutting_down(spa) || !metaslab_preload_enabled) {
                taskq_wait(mg->mg_taskq);
                return;
        }

        mutex_enter(&mg->mg_lock);

        /*
         * Load the next potential metaslabs
         */
        for (msp = avl_first(t); msp != NULL; msp = AVL_NEXT(t, msp)) {
                ASSERT3P(msp->ms_group, ==, mg);

                /*
                 * We preload only the maximum number of metaslabs specified
                 * by metaslab_preload_limit. If a metaslab is being forced
                 * to condense then we preload it too. This will ensure
                 * that force condensing happens in the next txg.
                 */
                if (++m > metaslab_preload_limit && !msp->ms_condense_wanted) {
                        continue;
                }

                VERIFY(taskq_dispatch(mg->mg_taskq, metaslab_preload,
                    msp, TQ_SLEEP) != (uintptr_t)NULL);
        }
        mutex_exit(&mg->mg_lock);
}

/*
 * Determine if the space map's on-disk footprint is past our tolerance
 * for inefficiency. We would like to use the following criteria to make
 * our decision:
 *
 * 1. The size of the space map object should not dramatically increase as a
 * result of writing out the free space range tree.
 *
 * 2. The minimal on-disk space map representation is zfs_condense_pct/100
 * times the size than the free space range tree representation
 * (i.e. zfs_condense_pct = 110 and in-core = 1MB, minimal = 1.1MB).
 *
 * 3. The on-disk size of the space map should actually decrease.
 *
 * Unfortunately, we cannot compute the on-disk size of the space map in this
 * context because we cannot accurately compute the effects of compression, etc.
 * Instead, we apply the heuristic described in the block comment for
 * zfs_metaslab_condense_block_threshold - we only condense if the space used
 * is greater than a threshold number of blocks.
 */
static boolean_t
metaslab_should_condense(metaslab_t *msp)
{
        space_map_t *sm = msp->ms_sm;
        vdev_t *vd = msp->ms_group->mg_vd;
        uint64_t vdev_blocksize = 1 << vd->vdev_ashift;
        uint64_t current_txg = spa_syncing_txg(vd->vdev_spa);

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT(msp->ms_loaded);

        /*
         * Allocations and frees in early passes are generally more space
         * efficient (in terms of blocks described in space map entries)
         * than the ones in later passes (e.g. we don't compress after
         * sync pass 5) and condensing a metaslab multiple times in a txg
         * could degrade performance.
         *
         * Thus we prefer condensing each metaslab at most once every txg at
         * the earliest sync pass possible. If a metaslab is eligible for
         * condensing again after being considered for condensing within the
         * same txg, it will hopefully be dirty in the next txg where it will
         * be condensed at an earlier pass.
         */
        if (msp->ms_condense_checked_txg == current_txg)
                return (B_FALSE);
        msp->ms_condense_checked_txg = current_txg;

        /*
         * We always condense metaslabs that are empty and metaslabs for
         * which a condense request has been made.
         */
        if (avl_is_empty(&msp->ms_allocatable_by_size) ||
            msp->ms_condense_wanted)
                return (B_TRUE);

        uint64_t object_size = space_map_length(msp->ms_sm);
        uint64_t optimal_size = space_map_estimate_optimal_size(sm,
            msp->ms_allocatable, SM_NO_VDEVID);

        dmu_object_info_t doi;
        dmu_object_info_from_db(sm->sm_dbuf, &doi);
        uint64_t record_size = MAX(doi.doi_data_block_size, vdev_blocksize);

        return (object_size >= (optimal_size * zfs_condense_pct / 100) &&
            object_size > zfs_metaslab_condense_block_threshold * record_size);
}

/*
 * Condense the on-disk space map representation to its minimized form.
 * The minimized form consists of a small number of allocations followed by
 * the entries of the free range tree.
 */
static void
metaslab_condense(metaslab_t *msp, uint64_t txg, dmu_tx_t *tx)
{
        range_tree_t *condense_tree;
        space_map_t *sm = msp->ms_sm;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT(msp->ms_loaded);

        zfs_dbgmsg("condensing: txg %llu, msp[%llu] %p, vdev id %llu, "
            "spa %s, smp size %llu, segments %lu, forcing condense=%s", txg,
            msp->ms_id, msp, msp->ms_group->mg_vd->vdev_id,
            msp->ms_group->mg_vd->vdev_spa->spa_name,
            space_map_length(msp->ms_sm),
            avl_numnodes(&msp->ms_allocatable->rt_root),
            msp->ms_condense_wanted ? "TRUE" : "FALSE");

        msp->ms_condense_wanted = B_FALSE;

        /*
         * Create an range tree that is 100% allocated. We remove segments
         * that have been freed in this txg, any deferred frees that exist,
         * and any allocation in the future. Removing segments should be
         * a relatively inexpensive operation since we expect these trees to
         * have a small number of nodes.
         */
        condense_tree = range_tree_create(NULL, NULL);
        range_tree_add(condense_tree, msp->ms_start, msp->ms_size);

        range_tree_walk(msp->ms_freeing, range_tree_remove, condense_tree);
        range_tree_walk(msp->ms_freed, range_tree_remove, condense_tree);

        for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                range_tree_walk(msp->ms_defer[t],
                    range_tree_remove, condense_tree);
        }

        for (int t = 1; t < TXG_CONCURRENT_STATES; t++) {
                range_tree_walk(msp->ms_allocating[(txg + t) & TXG_MASK],
                    range_tree_remove, condense_tree);
        }

        /*
         * We're about to drop the metaslab's lock thus allowing
         * other consumers to change it's content. Set the
         * metaslab's ms_condensing flag to ensure that
         * allocations on this metaslab do not occur while we're
         * in the middle of committing it to disk. This is only critical
         * for ms_allocatable as all other range trees use per txg
         * views of their content.
         */
        msp->ms_condensing = B_TRUE;

        mutex_exit(&msp->ms_lock);
        space_map_truncate(sm, zfs_metaslab_sm_blksz, tx);

        /*
         * While we would ideally like to create a space map representation
         * that consists only of allocation records, doing so can be
         * prohibitively expensive because the in-core free tree can be
         * large, and therefore computationally expensive to subtract
         * from the condense_tree. Instead we sync out two trees, a cheap
         * allocation only tree followed by the in-core free tree. While not
         * optimal, this is typically close to optimal, and much cheaper to
         * compute.
         */
        space_map_write(sm, condense_tree, SM_ALLOC, SM_NO_VDEVID, tx);
        range_tree_vacate(condense_tree, NULL, NULL);
        range_tree_destroy(condense_tree);

        space_map_write(sm, msp->ms_allocatable, SM_FREE, SM_NO_VDEVID, tx);
        mutex_enter(&msp->ms_lock);
        msp->ms_condensing = B_FALSE;
}

/*
 * Write a metaslab to disk in the context of the specified transaction group.
 */
void
metaslab_sync(metaslab_t *msp, uint64_t txg)
{
        metaslab_group_t *mg = msp->ms_group;
        vdev_t *vd = mg->mg_vd;
        spa_t *spa = vd->vdev_spa;
        objset_t *mos = spa_meta_objset(spa);
        range_tree_t *alloctree = msp->ms_allocating[txg & TXG_MASK];
        dmu_tx_t *tx;
        uint64_t object = space_map_object(msp->ms_sm);

        ASSERT(!vd->vdev_ishole);

        /*
         * This metaslab has just been added so there's no work to do now.
         */
        if (msp->ms_freeing == NULL) {
                ASSERT3P(alloctree, ==, NULL);
                return;
        }

        ASSERT3P(alloctree, !=, NULL);
        ASSERT3P(msp->ms_freeing, !=, NULL);
        ASSERT3P(msp->ms_freed, !=, NULL);
        ASSERT3P(msp->ms_checkpointing, !=, NULL);

        /*
         * Normally, we don't want to process a metaslab if there are no
         * allocations or frees to perform. However, if the metaslab is being
         * forced to condense and it's loaded, we need to let it through.
         */
        if (range_tree_is_empty(alloctree) &&
            range_tree_is_empty(msp->ms_freeing) &&
            range_tree_is_empty(msp->ms_checkpointing) &&
            !(msp->ms_loaded && msp->ms_condense_wanted))
                return;


        VERIFY(txg <= spa_final_dirty_txg(spa));

        /*
         * The only state that can actually be changing concurrently with
         * metaslab_sync() is the metaslab's ms_allocatable.  No other
         * thread can be modifying this txg's alloc, freeing,
         * freed, or space_map_phys_t.  We drop ms_lock whenever we
         * could call into the DMU, because the DMU can call down to us
         * (e.g. via zio_free()) at any time.
         *
         * The spa_vdev_remove_thread() can be reading metaslab state
         * concurrently, and it is locked out by the ms_sync_lock.  Note
         * that the ms_lock is insufficient for this, because it is dropped
         * by space_map_write().
         */
        tx = dmu_tx_create_assigned(spa_get_dsl(spa), txg);

        if (msp->ms_sm == NULL) {
                uint64_t new_object;

                new_object = space_map_alloc(mos, zfs_metaslab_sm_blksz, tx);
                VERIFY3U(new_object, !=, 0);

                VERIFY0(space_map_open(&msp->ms_sm, mos, new_object,
                    msp->ms_start, msp->ms_size, vd->vdev_ashift));
                ASSERT(msp->ms_sm != NULL);
        }

        if (!range_tree_is_empty(msp->ms_checkpointing) &&
            vd->vdev_checkpoint_sm == NULL) {
                ASSERT(spa_has_checkpoint(spa));

                uint64_t new_object = space_map_alloc(mos,
                    vdev_standard_sm_blksz, tx);
                VERIFY3U(new_object, !=, 0);

                VERIFY0(space_map_open(&vd->vdev_checkpoint_sm,
                    mos, new_object, 0, vd->vdev_asize, vd->vdev_ashift));
                ASSERT3P(vd->vdev_checkpoint_sm, !=, NULL);

                /*
                 * We save the space map object as an entry in vdev_top_zap
                 * so it can be retrieved when the pool is reopened after an
                 * export or through zdb.
                 */
                VERIFY0(zap_add(vd->vdev_spa->spa_meta_objset,
                    vd->vdev_top_zap, VDEV_TOP_ZAP_POOL_CHECKPOINT_SM,
                    sizeof (new_object), 1, &new_object, tx));
        }

        mutex_enter(&msp->ms_sync_lock);
        mutex_enter(&msp->ms_lock);

        /*
         * Note: metaslab_condense() clears the space map's histogram.
         * Therefore we must verify and remove this histogram before
         * condensing.
         */
        metaslab_group_histogram_verify(mg);
        metaslab_class_histogram_verify(mg->mg_class);
        metaslab_group_histogram_remove(mg, msp);

        if (msp->ms_loaded && metaslab_should_condense(msp)) {
                metaslab_condense(msp, txg, tx);
        } else {
                mutex_exit(&msp->ms_lock);
                space_map_write(msp->ms_sm, alloctree, SM_ALLOC,
                    SM_NO_VDEVID, tx);
                space_map_write(msp->ms_sm, msp->ms_freeing, SM_FREE,
                    SM_NO_VDEVID, tx);
                mutex_enter(&msp->ms_lock);
        }

        if (!range_tree_is_empty(msp->ms_checkpointing)) {
                ASSERT(spa_has_checkpoint(spa));
                ASSERT3P(vd->vdev_checkpoint_sm, !=, NULL);

                /*
                 * Since we are doing writes to disk and the ms_checkpointing
                 * tree won't be changing during that time, we drop the
                 * ms_lock while writing to the checkpoint space map.
                 */
                mutex_exit(&msp->ms_lock);
                space_map_write(vd->vdev_checkpoint_sm,
                    msp->ms_checkpointing, SM_FREE, SM_NO_VDEVID, tx);
                mutex_enter(&msp->ms_lock);
                space_map_update(vd->vdev_checkpoint_sm);

                spa->spa_checkpoint_info.sci_dspace +=
                    range_tree_space(msp->ms_checkpointing);
                vd->vdev_stat.vs_checkpoint_space +=
                    range_tree_space(msp->ms_checkpointing);
                ASSERT3U(vd->vdev_stat.vs_checkpoint_space, ==,
                    -vd->vdev_checkpoint_sm->sm_alloc);

                range_tree_vacate(msp->ms_checkpointing, NULL, NULL);
        }

        if (msp->ms_loaded) {
                /*
                 * When the space map is loaded, we have an accurate
                 * histogram in the range tree. This gives us an opportunity
                 * to bring the space map's histogram up-to-date so we clear
                 * it first before updating it.
                 */
                space_map_histogram_clear(msp->ms_sm);
                space_map_histogram_add(msp->ms_sm, msp->ms_allocatable, tx);

                /*
                 * Since we've cleared the histogram we need to add back
                 * any free space that has already been processed, plus
                 * any deferred space. This allows the on-disk histogram
                 * to accurately reflect all free space even if some space
                 * is not yet available for allocation (i.e. deferred).
                 */
                space_map_histogram_add(msp->ms_sm, msp->ms_freed, tx);

                /*
                 * Add back any deferred free space that has not been
                 * added back into the in-core free tree yet. This will
                 * ensure that we don't end up with a space map histogram
                 * that is completely empty unless the metaslab is fully
                 * allocated.
                 */
                for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                        space_map_histogram_add(msp->ms_sm,
                            msp->ms_defer[t], tx);
                }
        }

        /*
         * Always add the free space from this sync pass to the space
         * map histogram. We want to make sure that the on-disk histogram
         * accounts for all free space. If the space map is not loaded,
         * then we will lose some accuracy but will correct it the next
         * time we load the space map.
         */
        space_map_histogram_add(msp->ms_sm, msp->ms_freeing, tx);

        metaslab_group_histogram_add(mg, msp);
        metaslab_group_histogram_verify(mg);
        metaslab_class_histogram_verify(mg->mg_class);

        /*
         * For sync pass 1, we avoid traversing this txg's free range tree
         * and instead will just swap the pointers for freeing and
         * freed. We can safely do this since the freed_tree is
         * guaranteed to be empty on the initial pass.
         */
        if (spa_sync_pass(spa) == 1) {
                range_tree_swap(&msp->ms_freeing, &msp->ms_freed);
        } else {
                range_tree_vacate(msp->ms_freeing,
                    range_tree_add, msp->ms_freed);
        }
        range_tree_vacate(alloctree, NULL, NULL);

        ASSERT0(range_tree_space(msp->ms_allocating[txg & TXG_MASK]));
        ASSERT0(range_tree_space(msp->ms_allocating[TXG_CLEAN(txg)
            & TXG_MASK]));
        ASSERT0(range_tree_space(msp->ms_freeing));
        ASSERT0(range_tree_space(msp->ms_checkpointing));

        mutex_exit(&msp->ms_lock);

        if (object != space_map_object(msp->ms_sm)) {
                object = space_map_object(msp->ms_sm);
                dmu_write(mos, vd->vdev_ms_array, sizeof (uint64_t) *
                    msp->ms_id, sizeof (uint64_t), &object, tx);
        }
        mutex_exit(&msp->ms_sync_lock);
        dmu_tx_commit(tx);
}

/*
 * Called after a transaction group has completely synced to mark
 * all of the metaslab's free space as usable.
 */
void
metaslab_sync_done(metaslab_t *msp, uint64_t txg)
{
        metaslab_group_t *mg = msp->ms_group;
        vdev_t *vd = mg->mg_vd;
        spa_t *spa = vd->vdev_spa;
        range_tree_t **defer_tree;
        int64_t alloc_delta, defer_delta;
        boolean_t defer_allowed = B_TRUE;

        ASSERT(!vd->vdev_ishole);

        mutex_enter(&msp->ms_lock);

        /*
         * If this metaslab is just becoming available, initialize its
         * range trees and add its capacity to the vdev.
         */
        if (msp->ms_freed == NULL) {
                for (int t = 0; t < TXG_SIZE; t++) {
                        ASSERT(msp->ms_allocating[t] == NULL);

                        msp->ms_allocating[t] = range_tree_create(NULL, NULL);
                }

                ASSERT3P(msp->ms_freeing, ==, NULL);
                msp->ms_freeing = range_tree_create(NULL, NULL);

                ASSERT3P(msp->ms_freed, ==, NULL);
                msp->ms_freed = range_tree_create(NULL, NULL);

                for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                        ASSERT(msp->ms_defer[t] == NULL);

                        msp->ms_defer[t] = range_tree_create(NULL, NULL);
                }

                ASSERT3P(msp->ms_checkpointing, ==, NULL);
                msp->ms_checkpointing = range_tree_create(NULL, NULL);

                vdev_space_update(vd, 0, 0, msp->ms_size);
        }
        ASSERT0(range_tree_space(msp->ms_freeing));
        ASSERT0(range_tree_space(msp->ms_checkpointing));

        defer_tree = &msp->ms_defer[txg % TXG_DEFER_SIZE];

        uint64_t free_space = metaslab_class_get_space(spa_normal_class(spa)) -
            metaslab_class_get_alloc(spa_normal_class(spa));
        if (free_space <= spa_get_slop_space(spa) || vd->vdev_removing) {
                defer_allowed = B_FALSE;
        }

        defer_delta = 0;
        alloc_delta = space_map_alloc_delta(msp->ms_sm);
        if (defer_allowed) {
                defer_delta = range_tree_space(msp->ms_freed) -
                    range_tree_space(*defer_tree);
        } else {
                defer_delta -= range_tree_space(*defer_tree);
        }

        vdev_space_update(vd, alloc_delta + defer_delta, defer_delta, 0);

        /*
         * If there's a metaslab_load() in progress, wait for it to complete
         * so that we have a consistent view of the in-core space map.
         */
        metaslab_load_wait(msp);

        /*
         * Move the frees from the defer_tree back to the free
         * range tree (if it's loaded). Swap the freed_tree and
         * the defer_tree -- this is safe to do because we've
         * just emptied out the defer_tree.
         */
        range_tree_vacate(*defer_tree,
            msp->ms_loaded ? range_tree_add : NULL, msp->ms_allocatable);
        if (defer_allowed) {
                range_tree_swap(&msp->ms_freed, defer_tree);
        } else {
                range_tree_vacate(msp->ms_freed,
                    msp->ms_loaded ? range_tree_add : NULL,
                    msp->ms_allocatable);
        }
        space_map_update(msp->ms_sm);

        msp->ms_deferspace += defer_delta;
        ASSERT3S(msp->ms_deferspace, >=, 0);
        ASSERT3S(msp->ms_deferspace, <=, msp->ms_size);
        if (msp->ms_deferspace != 0) {
                /*
                 * Keep syncing this metaslab until all deferred frees
                 * are back in circulation.
                 */
                vdev_dirty(vd, VDD_METASLAB, msp, txg + 1);
        }

        /*
         * Calculate the new weights before unloading any metaslabs.
         * This will give us the most accurate weighting.
         */
        metaslab_group_sort(mg, msp, metaslab_weight(msp));

        /*
         * If the metaslab is loaded and we've not tried to load or allocate
         * from it in 'metaslab_unload_delay' txgs, then unload it.
         */
        if (msp->ms_loaded &&
            msp->ms_selected_txg + metaslab_unload_delay < txg) {
                for (int t = 1; t < TXG_CONCURRENT_STATES; t++) {
                        VERIFY0(range_tree_space(
                            msp->ms_allocating[(txg + t) & TXG_MASK]));
                }

                if (!metaslab_debug_unload)
                        metaslab_unload(msp);
        }

        ASSERT0(range_tree_space(msp->ms_allocating[txg & TXG_MASK]));
        ASSERT0(range_tree_space(msp->ms_freeing));
        ASSERT0(range_tree_space(msp->ms_freed));
        ASSERT0(range_tree_space(msp->ms_checkpointing));

        mutex_exit(&msp->ms_lock);
}

void
metaslab_sync_reassess(metaslab_group_t *mg)
{
        spa_t *spa = mg->mg_class->mc_spa;

        spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);
        metaslab_group_alloc_update(mg);
        mg->mg_fragmentation = metaslab_group_fragmentation(mg);

        /*
         * Preload the next potential metaslabs but only on active
         * metaslab groups. We can get into a state where the metaslab
         * is no longer active since we dirty metaslabs as we remove a
         * a device, thus potentially making the metaslab group eligible
         * for preloading.
         */
        if (mg->mg_activation_count > 0) {
                metaslab_group_preload(mg);
        }
        spa_config_exit(spa, SCL_ALLOC, FTAG);
}

static uint64_t
metaslab_distance(metaslab_t *msp, dva_t *dva)
{
        uint64_t ms_shift = msp->ms_group->mg_vd->vdev_ms_shift;
        uint64_t offset = DVA_GET_OFFSET(dva) >> ms_shift;
        uint64_t start = msp->ms_id;

        if (msp->ms_group->mg_vd->vdev_id != DVA_GET_VDEV(dva))
                return (1ULL << 63);

        if (offset < start)
                return ((start - offset) << ms_shift);
        if (offset > start)
                return ((offset - start) << ms_shift);
        return (0);
}

/*
 * ==========================================================================
 * Metaslab allocation tracing facility
 * ==========================================================================
 */
kstat_t *metaslab_trace_ksp;
kstat_named_t metaslab_trace_over_limit;

void
metaslab_alloc_trace_init(void)
{
        ASSERT(metaslab_alloc_trace_cache == NULL);
        metaslab_alloc_trace_cache = kmem_cache_create(
            "metaslab_alloc_trace_cache", sizeof (metaslab_alloc_trace_t),
            0, NULL, NULL, NULL, NULL, NULL, 0);
        metaslab_trace_ksp = kstat_create("zfs", 0, "metaslab_trace_stats",
            "misc", KSTAT_TYPE_NAMED, 1, KSTAT_FLAG_VIRTUAL);
        if (metaslab_trace_ksp != NULL) {
                metaslab_trace_ksp->ks_data = &metaslab_trace_over_limit;
                kstat_named_init(&metaslab_trace_over_limit,
                    "metaslab_trace_over_limit", KSTAT_DATA_UINT64);
                kstat_install(metaslab_trace_ksp);
        }
}

void
metaslab_alloc_trace_fini(void)
{
        if (metaslab_trace_ksp != NULL) {
                kstat_delete(metaslab_trace_ksp);
                metaslab_trace_ksp = NULL;
        }
        kmem_cache_destroy(metaslab_alloc_trace_cache);
        metaslab_alloc_trace_cache = NULL;
}

/*
 * Add an allocation trace element to the allocation tracing list.
 */
static void
metaslab_trace_add(zio_alloc_list_t *zal, metaslab_group_t *mg,
    metaslab_t *msp, uint64_t psize, uint32_t dva_id, uint64_t offset)
{
        if (!metaslab_trace_enabled)
                return;

        /*
         * When the tracing list reaches its maximum we remove
         * the second element in the list before adding a new one.
         * By removing the second element we preserve the original
         * entry as a clue to what allocations steps have already been
         * performed.
         */
        if (zal->zal_size == metaslab_trace_max_entries) {
                metaslab_alloc_trace_t *mat_next;
#ifdef DEBUG
                panic("too many entries in allocation list");
#endif
                atomic_inc_64(&metaslab_trace_over_limit.value.ui64);
                zal->zal_size--;
                mat_next = list_next(&zal->zal_list, list_head(&zal->zal_list));
                list_remove(&zal->zal_list, mat_next);
                kmem_cache_free(metaslab_alloc_trace_cache, mat_next);
        }

        metaslab_alloc_trace_t *mat =
            kmem_cache_alloc(metaslab_alloc_trace_cache, KM_SLEEP);
        list_link_init(&mat->mat_list_node);
        mat->mat_mg = mg;
        mat->mat_msp = msp;
        mat->mat_size = psize;
        mat->mat_dva_id = dva_id;
        mat->mat_offset = offset;
        mat->mat_weight = 0;

        if (msp != NULL)
                mat->mat_weight = msp->ms_weight;

        /*
         * The list is part of the zio so locking is not required. Only
         * a single thread will perform allocations for a given zio.
         */
        list_insert_tail(&zal->zal_list, mat);
        zal->zal_size++;

        ASSERT3U(zal->zal_size, <=, metaslab_trace_max_entries);
}

void
metaslab_trace_init(zio_alloc_list_t *zal)
{
        list_create(&zal->zal_list, sizeof (metaslab_alloc_trace_t),
            offsetof(metaslab_alloc_trace_t, mat_list_node));
        zal->zal_size = 0;
}

void
metaslab_trace_fini(zio_alloc_list_t *zal)
{
        metaslab_alloc_trace_t *mat;

        while ((mat = list_remove_head(&zal->zal_list)) != NULL)
                kmem_cache_free(metaslab_alloc_trace_cache, mat);
        list_destroy(&zal->zal_list);
        zal->zal_size = 0;
}

/*
 * ==========================================================================
 * Metaslab block operations
 * ==========================================================================
 */

static void
metaslab_group_alloc_increment(spa_t *spa, uint64_t vdev, void *tag, int flags)
{
        if (!(flags & METASLAB_ASYNC_ALLOC) ||
            flags & METASLAB_DONT_THROTTLE)
                return;

        metaslab_group_t *mg = vdev_lookup_top(spa, vdev)->vdev_mg;
        if (!mg->mg_class->mc_alloc_throttle_enabled)
                return;

        (void) refcount_add(&mg->mg_alloc_queue_depth, tag);
}

void
metaslab_group_alloc_decrement(spa_t *spa, uint64_t vdev, void *tag, int flags)
{
        if (!(flags & METASLAB_ASYNC_ALLOC) ||
            flags & METASLAB_DONT_THROTTLE)
                return;

        metaslab_group_t *mg = vdev_lookup_top(spa, vdev)->vdev_mg;
        if (!mg->mg_class->mc_alloc_throttle_enabled)
                return;

        (void) refcount_remove(&mg->mg_alloc_queue_depth, tag);
}

void
metaslab_group_alloc_verify(spa_t *spa, const blkptr_t *bp, void *tag)
{
#ifdef ZFS_DEBUG
        const dva_t *dva = bp->blk_dva;
        int ndvas = BP_GET_NDVAS(bp);

        for (int d = 0; d < ndvas; d++) {
                uint64_t vdev = DVA_GET_VDEV(&dva[d]);
                metaslab_group_t *mg = vdev_lookup_top(spa, vdev)->vdev_mg;
                VERIFY(refcount_not_held(&mg->mg_alloc_queue_depth, tag));
        }
#endif
}

static uint64_t
metaslab_block_alloc(metaslab_t *msp, uint64_t size, uint64_t txg)
{
        uint64_t start;
        range_tree_t *rt = msp->ms_allocatable;
        metaslab_class_t *mc = msp->ms_group->mg_class;

        VERIFY(!msp->ms_condensing);

        start = mc->mc_ops->msop_alloc(msp, size);
        if (start != -1ULL) {
                metaslab_group_t *mg = msp->ms_group;
                vdev_t *vd = mg->mg_vd;

                VERIFY0(P2PHASE(start, 1ULL << vd->vdev_ashift));
                VERIFY0(P2PHASE(size, 1ULL << vd->vdev_ashift));
                VERIFY3U(range_tree_space(rt) - size, <=, msp->ms_size);
                range_tree_remove(rt, start, size);

                if (range_tree_is_empty(msp->ms_allocating[txg & TXG_MASK]))
                        vdev_dirty(mg->mg_vd, VDD_METASLAB, msp, txg);

                range_tree_add(msp->ms_allocating[txg & TXG_MASK], start, size);

                /* Track the last successful allocation */
                msp->ms_alloc_txg = txg;
                metaslab_verify_space(msp, txg);
        }

        /*
         * Now that we've attempted the allocation we need to update the
         * metaslab's maximum block size since it may have changed.
         */
        msp->ms_max_size = metaslab_block_maxsize(msp);
        return (start);
}

static uint64_t
metaslab_group_alloc_normal(metaslab_group_t *mg, zio_alloc_list_t *zal,
    uint64_t asize, uint64_t txg, uint64_t min_distance, dva_t *dva, int d)
{
        metaslab_t *msp = NULL;
        uint64_t offset = -1ULL;
        uint64_t activation_weight;
        uint64_t target_distance;
        int i;

        activation_weight = METASLAB_WEIGHT_PRIMARY;
        for (i = 0; i < d; i++) {
                if (DVA_GET_VDEV(&dva[i]) == mg->mg_vd->vdev_id) {
                        activation_weight = METASLAB_WEIGHT_SECONDARY;
                        break;
                }
        }

        metaslab_t *search = kmem_alloc(sizeof (*search), KM_SLEEP);
        search->ms_weight = UINT64_MAX;
        search->ms_start = 0;
        for (;;) {
                boolean_t was_active;
                avl_tree_t *t = &mg->mg_metaslab_tree;
                avl_index_t idx;

                mutex_enter(&mg->mg_lock);

                /*
                 * Find the metaslab with the highest weight that is less
                 * than what we've already tried.  In the common case, this
                 * means that we will examine each metaslab at most once.
                 * Note that concurrent callers could reorder metaslabs
                 * by activation/passivation once we have dropped the mg_lock.
                 * If a metaslab is activated by another thread, and we fail
                 * to allocate from the metaslab we have selected, we may
                 * not try the newly-activated metaslab, and instead activate
                 * another metaslab.  This is not optimal, but generally
                 * does not cause any problems (a possible exception being
                 * if every metaslab is completely full except for the
                 * the newly-activated metaslab which we fail to examine).
                 */
                msp = avl_find(t, search, &idx);
                if (msp == NULL)
                        msp = avl_nearest(t, idx, AVL_AFTER);
                for (; msp != NULL; msp = AVL_NEXT(t, msp)) {

                        if (!metaslab_should_allocate(msp, asize)) {
                                metaslab_trace_add(zal, mg, msp, asize, d,
                                    TRACE_TOO_SMALL);
                                continue;
                        }

                        /*
                         * If the selected metaslab is condensing, skip it.
                         */
                        if (msp->ms_condensing)
                                continue;

                        was_active = msp->ms_weight & METASLAB_ACTIVE_MASK;
                        if (activation_weight == METASLAB_WEIGHT_PRIMARY)
                                break;

                        target_distance = min_distance +
                            (space_map_allocated(msp->ms_sm) != 0 ? 0 :
                            min_distance >> 1);

                        for (i = 0; i < d; i++) {
                                if (metaslab_distance(msp, &dva[i]) <
                                    target_distance)
                                        break;
                        }
                        if (i == d)
                                break;
                }
                mutex_exit(&mg->mg_lock);
                if (msp == NULL) {
                        kmem_free(search, sizeof (*search));
                        return (-1ULL);
                }
                search->ms_weight = msp->ms_weight;
                search->ms_start = msp->ms_start + 1;

                mutex_enter(&msp->ms_lock);

                /*
                 * Ensure that the metaslab we have selected is still
                 * capable of handling our request. It's possible that
                 * another thread may have changed the weight while we
                 * were blocked on the metaslab lock. We check the
                 * active status first to see if we need to reselect
                 * a new metaslab.
                 */
                if (was_active && !(msp->ms_weight & METASLAB_ACTIVE_MASK)) {
                        mutex_exit(&msp->ms_lock);
                        continue;
                }

                if ((msp->ms_weight & METASLAB_WEIGHT_SECONDARY) &&
                    activation_weight == METASLAB_WEIGHT_PRIMARY) {
                        metaslab_passivate(msp,
                            msp->ms_weight & ~METASLAB_ACTIVE_MASK);
                        mutex_exit(&msp->ms_lock);
                        continue;
                }

                if (metaslab_activate(msp, activation_weight) != 0) {
                        mutex_exit(&msp->ms_lock);
                        continue;
                }
                msp->ms_selected_txg = txg;

                /*
                 * Now that we have the lock, recheck to see if we should
                 * continue to use this metaslab for this allocation. The
                 * the metaslab is now loaded so metaslab_should_allocate() can
                 * accurately determine if the allocation attempt should
                 * proceed.
                 */
                if (!metaslab_should_allocate(msp, asize)) {
                        /* Passivate this metaslab and select a new one. */
                        metaslab_trace_add(zal, mg, msp, asize, d,
                            TRACE_TOO_SMALL);
                        goto next;
                }

                /*
                 * If this metaslab is currently condensing then pick again as
                 * we can't manipulate this metaslab until it's committed
                 * to disk.
                 */
                if (msp->ms_condensing) {
                        metaslab_trace_add(zal, mg, msp, asize, d,
                            TRACE_CONDENSING);
                        mutex_exit(&msp->ms_lock);
                        continue;
                }

                offset = metaslab_block_alloc(msp, asize, txg);
                metaslab_trace_add(zal, mg, msp, asize, d, offset);

                if (offset != -1ULL) {
                        /* Proactively passivate the metaslab, if needed */
                        metaslab_segment_may_passivate(msp);
                        break;
                }
next:
                ASSERT(msp->ms_loaded);

                /*
                 * We were unable to allocate from this metaslab so determine
                 * a new weight for this metaslab. Now that we have loaded
                 * the metaslab we can provide a better hint to the metaslab
                 * selector.
                 *
                 * For space-based metaslabs, we use the maximum block size.
                 * This information is only available when the metaslab
                 * is loaded and is more accurate than the generic free
                 * space weight that was calculated by metaslab_weight().
                 * This information allows us to quickly compare the maximum
                 * available allocation in the metaslab to the allocation
                 * size being requested.
                 *
                 * For segment-based metaslabs, determine the new weight
                 * based on the highest bucket in the range tree. We
                 * explicitly use the loaded segment weight (i.e. the range
                 * tree histogram) since it contains the space that is
                 * currently available for allocation and is accurate
                 * even within a sync pass.
                 */
                if (WEIGHT_IS_SPACEBASED(msp->ms_weight)) {
                        uint64_t weight = metaslab_block_maxsize(msp);
                        WEIGHT_SET_SPACEBASED(weight);
                        metaslab_passivate(msp, weight);
                } else {
                        metaslab_passivate(msp,
                            metaslab_weight_from_range_tree(msp));
                }

                /*
                 * We have just failed an allocation attempt, check
                 * that metaslab_should_allocate() agrees. Otherwise,
                 * we may end up in an infinite loop retrying the same
                 * metaslab.
                 */
                ASSERT(!metaslab_should_allocate(msp, asize));
                mutex_exit(&msp->ms_lock);
        }
        mutex_exit(&msp->ms_lock);
        kmem_free(search, sizeof (*search));
        return (offset);
}

static uint64_t
metaslab_group_alloc(metaslab_group_t *mg, zio_alloc_list_t *zal,
    uint64_t asize, uint64_t txg, uint64_t min_distance, dva_t *dva, int d)
{
        uint64_t offset;
        ASSERT(mg->mg_initialized);

        offset = metaslab_group_alloc_normal(mg, zal, asize, txg,
            min_distance, dva, d);

        mutex_enter(&mg->mg_lock);
        if (offset == -1ULL) {
                mg->mg_failed_allocations++;
                metaslab_trace_add(zal, mg, NULL, asize, d,
                    TRACE_GROUP_FAILURE);
                if (asize == SPA_GANGBLOCKSIZE) {
                        /*
                         * This metaslab group was unable to allocate
                         * the minimum gang block size so it must be out of
                         * space. We must notify the allocation throttle
                         * to start skipping allocation attempts to this
                         * metaslab group until more space becomes available.
                         * Note: this failure cannot be caused by the
                         * allocation throttle since the allocation throttle
                         * is only responsible for skipping devices and
                         * not failing block allocations.
                         */
                        mg->mg_no_free_space = B_TRUE;
                }
        }
        mg->mg_allocations++;
        mutex_exit(&mg->mg_lock);
        return (offset);
}

/*
 * If we have to write a ditto block (i.e. more than one DVA for a given BP)
 * on the same vdev as an existing DVA of this BP, then try to allocate it
 * at least (vdev_asize / (2 ^ ditto_same_vdev_distance_shift)) away from the
 * existing DVAs.
 */
int ditto_same_vdev_distance_shift = 3;

/*
 * Allocate a block for the specified i/o.
 */
int
metaslab_alloc_dva(spa_t *spa, metaslab_class_t *mc, uint64_t psize,
    dva_t *dva, int d, dva_t *hintdva, uint64_t txg, int flags,
    zio_alloc_list_t *zal)
{
        metaslab_group_t *mg, *rotor;
        vdev_t *vd;
        boolean_t try_hard = B_FALSE;

        ASSERT(!DVA_IS_VALID(&dva[d]));

        /*
         * For testing, make some blocks above a certain size be gang blocks.
         */
        if (psize >= metaslab_force_ganging && (ddi_get_lbolt() & 3) == 0) {
                metaslab_trace_add(zal, NULL, NULL, psize, d, TRACE_FORCE_GANG);
                return (SET_ERROR(ENOSPC));
        }

        /*
         * Start at the rotor and loop through all mgs until we find something.
         * Note that there's no locking on mc_rotor or mc_aliquot because
         * nothing actually breaks if we miss a few updates -- we just won't
         * allocate quite as evenly.  It all balances out over time.
         *
         * If we are doing ditto or log blocks, try to spread them across
         * consecutive vdevs.  If we're forced to reuse a vdev before we've
         * allocated all of our ditto blocks, then try and spread them out on
         * that vdev as much as possible.  If it turns out to not be possible,
         * gradually lower our standards until anything becomes acceptable.
         * Also, allocating on consecutive vdevs (as opposed to random vdevs)
         * gives us hope of containing our fault domains to something we're
         * able to reason about.  Otherwise, any two top-level vdev failures
         * will guarantee the loss of data.  With consecutive allocation,
         * only two adjacent top-level vdev failures will result in data loss.
         *
         * If we are doing gang blocks (hintdva is non-NULL), try to keep
         * ourselves on the same vdev as our gang block header.  That
         * way, we can hope for locality in vdev_cache, plus it makes our
         * fault domains something tractable.
         */
        if (hintdva) {
                vd = vdev_lookup_top(spa, DVA_GET_VDEV(&hintdva[d]));

                /*
                 * It's possible the vdev we're using as the hint no
                 * longer exists or its mg has been closed (e.g. by
                 * device removal).  Consult the rotor when
                 * all else fails.
                 */
                if (vd != NULL && vd->vdev_mg != NULL) {
                        mg = vd->vdev_mg;

                        if (flags & METASLAB_HINTBP_AVOID &&
                            mg->mg_next != NULL)
                                mg = mg->mg_next;
                } else {
                        mg = mc->mc_rotor;
                }
        } else if (d != 0) {
                vd = vdev_lookup_top(spa, DVA_GET_VDEV(&dva[d - 1]));
                mg = vd->vdev_mg->mg_next;
        } else {
                mg = mc->mc_rotor;
        }

        /*
         * If the hint put us into the wrong metaslab class, or into a
         * metaslab group that has been passivated, just follow the rotor.
         */
        if (mg->mg_class != mc || mg->mg_activation_count <= 0)
                mg = mc->mc_rotor;

        rotor = mg;
top:
        do {
                boolean_t allocatable;

                ASSERT(mg->mg_activation_count == 1);
                vd = mg->mg_vd;

                /*
                 * Don't allocate from faulted devices.
                 */
                if (try_hard) {
                        spa_config_enter(spa, SCL_ZIO, FTAG, RW_READER);
                        allocatable = vdev_allocatable(vd);
                        spa_config_exit(spa, SCL_ZIO, FTAG);
                } else {
                        allocatable = vdev_allocatable(vd);
                }

                /*
                 * Determine if the selected metaslab group is eligible
                 * for allocations. If we're ganging then don't allow
                 * this metaslab group to skip allocations since that would
                 * inadvertently return ENOSPC and suspend the pool
                 * even though space is still available.
                 */
                if (allocatable && !GANG_ALLOCATION(flags) && !try_hard) {
                        allocatable = metaslab_group_allocatable(mg, rotor,
                            psize);
                }

                if (!allocatable) {
                        metaslab_trace_add(zal, mg, NULL, psize, d,
                            TRACE_NOT_ALLOCATABLE);
                        goto next;
                }

                ASSERT(mg->mg_initialized);

                /*
                 * Avoid writing single-copy data to a failing,
                 * non-redundant vdev, unless we've already tried all
                 * other vdevs.
                 */
                if ((vd->vdev_stat.vs_write_errors > 0 ||
                    vd->vdev_state < VDEV_STATE_HEALTHY) &&
                    d == 0 && !try_hard && vd->vdev_children == 0) {
                        metaslab_trace_add(zal, mg, NULL, psize, d,
                            TRACE_VDEV_ERROR);
                        goto next;
                }

                ASSERT(mg->mg_class == mc);

                /*
                 * If we don't need to try hard, then require that the
                 * block be 1/8th of the device away from any other DVAs
                 * in this BP.  If we are trying hard, allow any offset
                 * to be used (distance=0).
                 */
                uint64_t distance = 0;
                if (!try_hard) {
                        distance = vd->vdev_asize >>
                            ditto_same_vdev_distance_shift;
                        if (distance <= (1ULL << vd->vdev_ms_shift))
                                distance = 0;
                }

                uint64_t asize = vdev_psize_to_asize(vd, psize);
                ASSERT(P2PHASE(asize, 1ULL << vd->vdev_ashift) == 0);

                uint64_t offset = metaslab_group_alloc(mg, zal, asize, txg,
                    distance, dva, d);

                if (offset != -1ULL) {
                        /*
                         * If we've just selected this metaslab group,
                         * figure out whether the corresponding vdev is
                         * over- or under-used relative to the pool,
                         * and set an allocation bias to even it out.
                         */
                        if (mc->mc_aliquot == 0 && metaslab_bias_enabled) {
                                vdev_stat_t *vs = &vd->vdev_stat;
                                int64_t vu, cu;

                                vu = (vs->vs_alloc * 100) / (vs->vs_space + 1);
                                cu = (mc->mc_alloc * 100) / (mc->mc_space + 1);

                                /*
                                 * Calculate how much more or less we should
                                 * try to allocate from this device during
                                 * this iteration around the rotor.
                                 * For example, if a device is 80% full
                                 * and the pool is 20% full then we should
                                 * reduce allocations by 60% on this device.
                                 *
                                 * mg_bias = (20 - 80) * 512K / 100 = -307K
                                 *
                                 * This reduces allocations by 307K for this
                                 * iteration.
                                 */
                                mg->mg_bias = ((cu - vu) *
                                    (int64_t)mg->mg_aliquot) / 100;
                        } else if (!metaslab_bias_enabled) {
                                mg->mg_bias = 0;
                        }

                        if (atomic_add_64_nv(&mc->mc_aliquot, asize) >=
                            mg->mg_aliquot + mg->mg_bias) {
                                mc->mc_rotor = mg->mg_next;
                                mc->mc_aliquot = 0;
                        }

                        DVA_SET_VDEV(&dva[d], vd->vdev_id);
                        DVA_SET_OFFSET(&dva[d], offset);
                        DVA_SET_GANG(&dva[d], !!(flags & METASLAB_GANG_HEADER));
                        DVA_SET_ASIZE(&dva[d], asize);

                        return (0);
                }
next:
                mc->mc_rotor = mg->mg_next;
                mc->mc_aliquot = 0;
        } while ((mg = mg->mg_next) != rotor);

        /*
         * If we haven't tried hard, do so now.
         */
        if (!try_hard) {
                try_hard = B_TRUE;
                goto top;
        }

        bzero(&dva[d], sizeof (dva_t));

        metaslab_trace_add(zal, rotor, NULL, psize, d, TRACE_ENOSPC);
        return (SET_ERROR(ENOSPC));
}

void
metaslab_free_concrete(vdev_t *vd, uint64_t offset, uint64_t asize,
    boolean_t checkpoint)
{
        metaslab_t *msp;
        spa_t *spa = vd->vdev_spa;

        ASSERT(vdev_is_concrete(vd));
        ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);
        ASSERT3U(offset >> vd->vdev_ms_shift, <, vd->vdev_ms_count);

        msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];

        VERIFY(!msp->ms_condensing);
        VERIFY3U(offset, >=, msp->ms_start);
        VERIFY3U(offset + asize, <=, msp->ms_start + msp->ms_size);
        VERIFY0(P2PHASE(offset, 1ULL << vd->vdev_ashift));
        VERIFY0(P2PHASE(asize, 1ULL << vd->vdev_ashift));

        metaslab_check_free_impl(vd, offset, asize);

        mutex_enter(&msp->ms_lock);
        if (range_tree_is_empty(msp->ms_freeing) &&
            range_tree_is_empty(msp->ms_checkpointing)) {
                vdev_dirty(vd, VDD_METASLAB, msp, spa_syncing_txg(spa));
        }

        if (checkpoint) {
                ASSERT(spa_has_checkpoint(spa));
                range_tree_add(msp->ms_checkpointing, offset, asize);
        } else {
                range_tree_add(msp->ms_freeing, offset, asize);
        }
        mutex_exit(&msp->ms_lock);
}

/* ARGSUSED */
void
metaslab_free_impl_cb(uint64_t inner_offset, vdev_t *vd, uint64_t offset,
    uint64_t size, void *arg)
{
        boolean_t *checkpoint = arg;

        ASSERT3P(checkpoint, !=, NULL);

        if (vd->vdev_ops->vdev_op_remap != NULL)
                vdev_indirect_mark_obsolete(vd, offset, size);
        else
                metaslab_free_impl(vd, offset, size, *checkpoint);
}

static void
metaslab_free_impl(vdev_t *vd, uint64_t offset, uint64_t size,
    boolean_t checkpoint)
{
        spa_t *spa = vd->vdev_spa;

        ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);

        if (spa_syncing_txg(spa) > spa_freeze_txg(spa))
                return;

        if (spa->spa_vdev_removal != NULL &&
            spa->spa_vdev_removal->svr_vdev == vd &&
            vdev_is_concrete(vd)) {
                /*
                 * Note: we check if the vdev is concrete because when
                 * we complete the removal, we first change the vdev to be
                 * an indirect vdev (in open context), and then (in syncing
                 * context) clear spa_vdev_removal.
                 */
                free_from_removing_vdev(vd, offset, size);
        } else if (vd->vdev_ops->vdev_op_remap != NULL) {
                vdev_indirect_mark_obsolete(vd, offset, size);
                vd->vdev_ops->vdev_op_remap(vd, offset, size,
                    metaslab_free_impl_cb, &checkpoint);
        } else {
                metaslab_free_concrete(vd, offset, size, checkpoint);
        }
}

typedef struct remap_blkptr_cb_arg {
        blkptr_t *rbca_bp;
        spa_remap_cb_t rbca_cb;
        vdev_t *rbca_remap_vd;
        uint64_t rbca_remap_offset;
        void *rbca_cb_arg;
} remap_blkptr_cb_arg_t;

void
remap_blkptr_cb(uint64_t inner_offset, vdev_t *vd, uint64_t offset,
    uint64_t size, void *arg)
{
        remap_blkptr_cb_arg_t *rbca = arg;
        blkptr_t *bp = rbca->rbca_bp;

        /* We can not remap split blocks. */
        if (size != DVA_GET_ASIZE(&bp->blk_dva[0]))
                return;
        ASSERT0(inner_offset);

        if (rbca->rbca_cb != NULL) {
                /*
                 * At this point we know that we are not handling split
                 * blocks and we invoke the callback on the previous
                 * vdev which must be indirect.
                 */
                ASSERT3P(rbca->rbca_remap_vd->vdev_ops, ==, &vdev_indirect_ops);

                rbca->rbca_cb(rbca->rbca_remap_vd->vdev_id,
                    rbca->rbca_remap_offset, size, rbca->rbca_cb_arg);

                /* set up remap_blkptr_cb_arg for the next call */
                rbca->rbca_remap_vd = vd;
                rbca->rbca_remap_offset = offset;
        }

        /*
         * The phys birth time is that of dva[0].  This ensures that we know
         * when each dva was written, so that resilver can determine which
         * blocks need to be scrubbed (i.e. those written during the time
         * the vdev was offline).  It also ensures that the key used in
         * the ARC hash table is unique (i.e. dva[0] + phys_birth).  If
         * we didn't change the phys_birth, a lookup in the ARC for a
         * remapped BP could find the data that was previously stored at
         * this vdev + offset.
         */
        vdev_t *oldvd = vdev_lookup_top(vd->vdev_spa,
            DVA_GET_VDEV(&bp->blk_dva[0]));
        vdev_indirect_births_t *vib = oldvd->vdev_indirect_births;
        bp->blk_phys_birth = vdev_indirect_births_physbirth(vib,
            DVA_GET_OFFSET(&bp->blk_dva[0]), DVA_GET_ASIZE(&bp->blk_dva[0]));

        DVA_SET_VDEV(&bp->blk_dva[0], vd->vdev_id);
        DVA_SET_OFFSET(&bp->blk_dva[0], offset);
}

/*
 * If the block pointer contains any indirect DVAs, modify them to refer to
 * concrete DVAs.  Note that this will sometimes not be possible, leaving
 * the indirect DVA in place.  This happens if the indirect DVA spans multiple
 * segments in the mapping (i.e. it is a "split block").
 *
 * If the BP was remapped, calls the callback on the original dva (note the
 * callback can be called multiple times if the original indirect DVA refers
 * to another indirect DVA, etc).
 *
 * Returns TRUE if the BP was remapped.
 */
boolean_t
spa_remap_blkptr(spa_t *spa, blkptr_t *bp, spa_remap_cb_t callback, void *arg)
{
        remap_blkptr_cb_arg_t rbca;

        if (!zfs_remap_blkptr_enable)
                return (B_FALSE);

        if (!spa_feature_is_enabled(spa, SPA_FEATURE_OBSOLETE_COUNTS))
                return (B_FALSE);

        /*
         * Dedup BP's can not be remapped, because ddt_phys_select() depends
         * on DVA[0] being the same in the BP as in the DDT (dedup table).
         */
        if (BP_GET_DEDUP(bp))
                return (B_FALSE);

        /*
         * Gang blocks can not be remapped, because
         * zio_checksum_gang_verifier() depends on the DVA[0] that's in
         * the BP used to read the gang block header (GBH) being the same
         * as the DVA[0] that we allocated for the GBH.
         */
        if (BP_IS_GANG(bp))
                return (B_FALSE);

        /*
         * Embedded BP's have no DVA to remap.
         */
        if (BP_GET_NDVAS(bp) < 1)
                return (B_FALSE);

        /*
         * Note: we only remap dva[0].  If we remapped other dvas, we
         * would no longer know what their phys birth txg is.
         */
        dva_t *dva = &bp->blk_dva[0];

        uint64_t offset = DVA_GET_OFFSET(dva);
        uint64_t size = DVA_GET_ASIZE(dva);
        vdev_t *vd = vdev_lookup_top(spa, DVA_GET_VDEV(dva));

        if (vd->vdev_ops->vdev_op_remap == NULL)
                return (B_FALSE);

        rbca.rbca_bp = bp;
        rbca.rbca_cb = callback;
        rbca.rbca_remap_vd = vd;
        rbca.rbca_remap_offset = offset;
        rbca.rbca_cb_arg = arg;

        /*
         * remap_blkptr_cb() will be called in order for each level of
         * indirection, until a concrete vdev is reached or a split block is
         * encountered. old_vd and old_offset are updated within the callback
         * as we go from the one indirect vdev to the next one (either concrete
         * or indirect again) in that order.
         */
        vd->vdev_ops->vdev_op_remap(vd, offset, size, remap_blkptr_cb, &rbca);

        /* Check if the DVA wasn't remapped because it is a split block */
        if (DVA_GET_VDEV(&rbca.rbca_bp->blk_dva[0]) == vd->vdev_id)
                return (B_FALSE);

        return (B_TRUE);
}

/*
 * Undo the allocation of a DVA which happened in the given transaction group.
 */
void
metaslab_unalloc_dva(spa_t *spa, const dva_t *dva, uint64_t txg)
{
        metaslab_t *msp;
        vdev_t *vd;
        uint64_t vdev = DVA_GET_VDEV(dva);
        uint64_t offset = DVA_GET_OFFSET(dva);
        uint64_t size = DVA_GET_ASIZE(dva);

        ASSERT(DVA_IS_VALID(dva));
        ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);

        if (txg > spa_freeze_txg(spa))
                return;

        if ((vd = vdev_lookup_top(spa, vdev)) == NULL ||
            (offset >> vd->vdev_ms_shift) >= vd->vdev_ms_count) {
                cmn_err(CE_WARN, "metaslab_free_dva(): bad DVA %llu:%llu",
                    (u_longlong_t)vdev, (u_longlong_t)offset);
                ASSERT(0);
                return;
        }

        ASSERT(!vd->vdev_removing);
        ASSERT(vdev_is_concrete(vd));
        ASSERT0(vd->vdev_indirect_config.vic_mapping_object);
        ASSERT3P(vd->vdev_indirect_mapping, ==, NULL);

        if (DVA_GET_GANG(dva))
                size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);

        msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];

        mutex_enter(&msp->ms_lock);
        range_tree_remove(msp->ms_allocating[txg & TXG_MASK],
            offset, size);

        VERIFY(!msp->ms_condensing);
        VERIFY3U(offset, >=, msp->ms_start);
        VERIFY3U(offset + size, <=, msp->ms_start + msp->ms_size);
        VERIFY3U(range_tree_space(msp->ms_allocatable) + size, <=,
            msp->ms_size);
        VERIFY0(P2PHASE(offset, 1ULL << vd->vdev_ashift));
        VERIFY0(P2PHASE(size, 1ULL << vd->vdev_ashift));
        range_tree_add(msp->ms_allocatable, offset, size);
        mutex_exit(&msp->ms_lock);
}

/*
 * Free the block represented by the given DVA.
 */
void
metaslab_free_dva(spa_t *spa, const dva_t *dva, boolean_t checkpoint)
{
        uint64_t vdev = DVA_GET_VDEV(dva);
        uint64_t offset = DVA_GET_OFFSET(dva);
        uint64_t size = DVA_GET_ASIZE(dva);
        vdev_t *vd = vdev_lookup_top(spa, vdev);

        ASSERT(DVA_IS_VALID(dva));
        ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);

        if (DVA_GET_GANG(dva)) {
                size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);
        }

        metaslab_free_impl(vd, offset, size, checkpoint);
}

/*
 * Reserve some allocation slots. The reservation system must be called
 * before we call into the allocator. If there aren't any available slots
 * then the I/O will be throttled until an I/O completes and its slots are
 * freed up. The function returns true if it was successful in placing
 * the reservation.
 */
boolean_t
metaslab_class_throttle_reserve(metaslab_class_t *mc, int slots, zio_t *zio,
    int flags)
{
        uint64_t available_slots = 0;
        boolean_t slot_reserved = B_FALSE;

        ASSERT(mc->mc_alloc_throttle_enabled);
        mutex_enter(&mc->mc_lock);

        uint64_t reserved_slots = refcount_count(&mc->mc_alloc_slots);
        if (reserved_slots < mc->mc_alloc_max_slots)
                available_slots = mc->mc_alloc_max_slots - reserved_slots;

        if (slots <= available_slots || GANG_ALLOCATION(flags)) {
                /*
                 * We reserve the slots individually so that we can unreserve
                 * them individually when an I/O completes.
                 */
                for (int d = 0; d < slots; d++) {
                        reserved_slots = refcount_add(&mc->mc_alloc_slots, zio);
                }
                zio->io_flags |= ZIO_FLAG_IO_ALLOCATING;
                slot_reserved = B_TRUE;
        }

        mutex_exit(&mc->mc_lock);
        return (slot_reserved);
}

void
metaslab_class_throttle_unreserve(metaslab_class_t *mc, int slots, zio_t *zio)
{
        ASSERT(mc->mc_alloc_throttle_enabled);
        mutex_enter(&mc->mc_lock);
        for (int d = 0; d < slots; d++) {
                (void) refcount_remove(&mc->mc_alloc_slots, zio);
        }
        mutex_exit(&mc->mc_lock);
}

static int
metaslab_claim_concrete(vdev_t *vd, uint64_t offset, uint64_t size,
    uint64_t txg)
{
        metaslab_t *msp;
        spa_t *spa = vd->vdev_spa;
        int error = 0;

        if (offset >> vd->vdev_ms_shift >= vd->vdev_ms_count)
                return (ENXIO);

        ASSERT3P(vd->vdev_ms, !=, NULL);
        msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];

        mutex_enter(&msp->ms_lock);

        if ((txg != 0 && spa_writeable(spa)) || !msp->ms_loaded)
                error = metaslab_activate(msp, METASLAB_WEIGHT_SECONDARY);

        if (error == 0 &&
            !range_tree_contains(msp->ms_allocatable, offset, size))
                error = SET_ERROR(ENOENT);

        if (error || txg == 0) {        /* txg == 0 indicates dry run */
                mutex_exit(&msp->ms_lock);
                return (error);
        }

        VERIFY(!msp->ms_condensing);
        VERIFY0(P2PHASE(offset, 1ULL << vd->vdev_ashift));
        VERIFY0(P2PHASE(size, 1ULL << vd->vdev_ashift));
        VERIFY3U(range_tree_space(msp->ms_allocatable) - size, <=,
            msp->ms_size);
        range_tree_remove(msp->ms_allocatable, offset, size);

        if (spa_writeable(spa)) {       /* don't dirty if we're zdb(8) */
                if (range_tree_is_empty(msp->ms_allocating[txg & TXG_MASK]))
                        vdev_dirty(vd, VDD_METASLAB, msp, txg);
                range_tree_add(msp->ms_allocating[txg & TXG_MASK],
                    offset, size);
        }

        mutex_exit(&msp->ms_lock);

        return (0);
}

typedef struct metaslab_claim_cb_arg_t {
        uint64_t        mcca_txg;
        int             mcca_error;
} metaslab_claim_cb_arg_t;

/* ARGSUSED */
static void
metaslab_claim_impl_cb(uint64_t inner_offset, vdev_t *vd, uint64_t offset,
    uint64_t size, void *arg)
{
        metaslab_claim_cb_arg_t *mcca_arg = arg;

        if (mcca_arg->mcca_error == 0) {
                mcca_arg->mcca_error = metaslab_claim_concrete(vd, offset,
                    size, mcca_arg->mcca_txg);
        }
}

int
metaslab_claim_impl(vdev_t *vd, uint64_t offset, uint64_t size, uint64_t txg)
{
        if (vd->vdev_ops->vdev_op_remap != NULL) {
                metaslab_claim_cb_arg_t arg;

                /*
                 * Only zdb(8) can claim on indirect vdevs.  This is used
                 * to detect leaks of mapped space (that are not accounted
                 * for in the obsolete counts, spacemap, or bpobj).
                 */
                ASSERT(!spa_writeable(vd->vdev_spa));
                arg.mcca_error = 0;
                arg.mcca_txg = txg;

                vd->vdev_ops->vdev_op_remap(vd, offset, size,
                    metaslab_claim_impl_cb, &arg);

                if (arg.mcca_error == 0) {
                        arg.mcca_error = metaslab_claim_concrete(vd,
                            offset, size, txg);
                }
                return (arg.mcca_error);
        } else {
                return (metaslab_claim_concrete(vd, offset, size, txg));
        }
}

/*
 * Intent log support: upon opening the pool after a crash, notify the SPA
 * of blocks that the intent log has allocated for immediate write, but
 * which are still considered free by the SPA because the last transaction
 * group didn't commit yet.
 */
static int
metaslab_claim_dva(spa_t *spa, const dva_t *dva, uint64_t txg)
{
        uint64_t vdev = DVA_GET_VDEV(dva);
        uint64_t offset = DVA_GET_OFFSET(dva);
        uint64_t size = DVA_GET_ASIZE(dva);
        vdev_t *vd;

        if ((vd = vdev_lookup_top(spa, vdev)) == NULL) {
                return (SET_ERROR(ENXIO));
        }

        ASSERT(DVA_IS_VALID(dva));

        if (DVA_GET_GANG(dva))
                size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);

        return (metaslab_claim_impl(vd, offset, size, txg));
}

int
metaslab_alloc(spa_t *spa, metaslab_class_t *mc, uint64_t psize, blkptr_t *bp,
    int ndvas, uint64_t txg, blkptr_t *hintbp, int flags,
    zio_alloc_list_t *zal, zio_t *zio)
{
        dva_t *dva = bp->blk_dva;
        dva_t *hintdva = hintbp->blk_dva;
        int error = 0;

        ASSERT(bp->blk_birth == 0);
        ASSERT(BP_PHYSICAL_BIRTH(bp) == 0);

        spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);

        if (mc->mc_rotor == NULL) {     /* no vdevs in this class */
                spa_config_exit(spa, SCL_ALLOC, FTAG);
                return (SET_ERROR(ENOSPC));
        }

        ASSERT(ndvas > 0 && ndvas <= spa_max_replication(spa));
        ASSERT(BP_GET_NDVAS(bp) == 0);
        ASSERT(hintbp == NULL || ndvas <= BP_GET_NDVAS(hintbp));
        ASSERT3P(zal, !=, NULL);

        for (int d = 0; d < ndvas; d++) {
                error = metaslab_alloc_dva(spa, mc, psize, dva, d, hintdva,
                    txg, flags, zal);
                if (error != 0) {
                        for (d--; d >= 0; d--) {
                                metaslab_unalloc_dva(spa, &dva[d], txg);
                                metaslab_group_alloc_decrement(spa,
                                    DVA_GET_VDEV(&dva[d]), zio, flags);
                                bzero(&dva[d], sizeof (dva_t));
                        }
                        spa_config_exit(spa, SCL_ALLOC, FTAG);
                        return (error);
                } else {
                        /*
                         * Update the metaslab group's queue depth
                         * based on the newly allocated dva.
                         */
                        metaslab_group_alloc_increment(spa,
                            DVA_GET_VDEV(&dva[d]), zio, flags);
                }

        }
        ASSERT(error == 0);
        ASSERT(BP_GET_NDVAS(bp) == ndvas);

        spa_config_exit(spa, SCL_ALLOC, FTAG);

        BP_SET_BIRTH(bp, txg, txg);

        return (0);
}

void
metaslab_free(spa_t *spa, const blkptr_t *bp, uint64_t txg, boolean_t now)
{
        const dva_t *dva = bp->blk_dva;
        int ndvas = BP_GET_NDVAS(bp);

        ASSERT(!BP_IS_HOLE(bp));
        ASSERT(!now || bp->blk_birth >= spa_syncing_txg(spa));

        /*
         * If we have a checkpoint for the pool we need to make sure that
         * the blocks that we free that are part of the checkpoint won't be
         * reused until the checkpoint is discarded or we revert to it.
         *
         * The checkpoint flag is passed down the metaslab_free code path
         * and is set whenever we want to add a block to the checkpoint's
         * accounting. That is, we "checkpoint" blocks that existed at the
         * time the checkpoint was created and are therefore referenced by
         * the checkpointed uberblock.
         *
         * Note that, we don't checkpoint any blocks if the current
         * syncing txg <= spa_checkpoint_txg. We want these frees to sync
         * normally as they will be referenced by the checkpointed uberblock.
         */
        boolean_t checkpoint = B_FALSE;
        if (bp->blk_birth <= spa->spa_checkpoint_txg &&
            spa_syncing_txg(spa) > spa->spa_checkpoint_txg) {
                /*
                 * At this point, if the block is part of the checkpoint
                 * there is no way it was created in the current txg.
                 */
                ASSERT(!now);
                ASSERT3U(spa_syncing_txg(spa), ==, txg);
                checkpoint = B_TRUE;
        }

        spa_config_enter(spa, SCL_FREE, FTAG, RW_READER);

        for (int d = 0; d < ndvas; d++) {
                if (now) {
                        metaslab_unalloc_dva(spa, &dva[d], txg);
                } else {
                        ASSERT3U(txg, ==, spa_syncing_txg(spa));
                        metaslab_free_dva(spa, &dva[d], checkpoint);
                }
        }

        spa_config_exit(spa, SCL_FREE, FTAG);
}

int
metaslab_claim(spa_t *spa, const blkptr_t *bp, uint64_t txg)
{
        const dva_t *dva = bp->blk_dva;
        int ndvas = BP_GET_NDVAS(bp);
        int error = 0;

        ASSERT(!BP_IS_HOLE(bp));

        if (txg != 0) {
                /*
                 * First do a dry run to make sure all DVAs are claimable,
                 * so we don't have to unwind from partial failures below.
                 */
                if ((error = metaslab_claim(spa, bp, 0)) != 0)
                        return (error);
        }

        spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);

        for (int d = 0; d < ndvas; d++)
                if ((error = metaslab_claim_dva(spa, &dva[d], txg)) != 0)
                        break;

        spa_config_exit(spa, SCL_ALLOC, FTAG);

        ASSERT(error == 0 || txg == 0);

        return (error);
}

/* ARGSUSED */
static void
metaslab_check_free_impl_cb(uint64_t inner, vdev_t *vd, uint64_t offset,
    uint64_t size, void *arg)
{
        if (vd->vdev_ops == &vdev_indirect_ops)
                return;

        metaslab_check_free_impl(vd, offset, size);
}

static void
metaslab_check_free_impl(vdev_t *vd, uint64_t offset, uint64_t size)
{
        metaslab_t *msp;
        spa_t *spa = vd->vdev_spa;

        if ((zfs_flags & ZFS_DEBUG_ZIO_FREE) == 0)
                return;

        if (vd->vdev_ops->vdev_op_remap != NULL) {
                vd->vdev_ops->vdev_op_remap(vd, offset, size,
                    metaslab_check_free_impl_cb, NULL);
                return;
        }

        ASSERT(vdev_is_concrete(vd));
        ASSERT3U(offset >> vd->vdev_ms_shift, <, vd->vdev_ms_count);
        ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);

        msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];

        mutex_enter(&msp->ms_lock);
        if (msp->ms_loaded)
                range_tree_verify(msp->ms_allocatable, offset, size);

        range_tree_verify(msp->ms_freeing, offset, size);
        range_tree_verify(msp->ms_checkpointing, offset, size);
        range_tree_verify(msp->ms_freed, offset, size);
        for (int j = 0; j < TXG_DEFER_SIZE; j++)
                range_tree_verify(msp->ms_defer[j], offset, size);
        mutex_exit(&msp->ms_lock);
}

void
metaslab_check_free(spa_t *spa, const blkptr_t *bp)
{
        if ((zfs_flags & ZFS_DEBUG_ZIO_FREE) == 0)
                return;

        spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
        for (int i = 0; i < BP_GET_NDVAS(bp); i++) {
                uint64_t vdev = DVA_GET_VDEV(&bp->blk_dva[i]);
                vdev_t *vd = vdev_lookup_top(spa, vdev);
                uint64_t offset = DVA_GET_OFFSET(&bp->blk_dva[i]);
                uint64_t size = DVA_GET_ASIZE(&bp->blk_dva[i]);

                if (DVA_GET_GANG(&bp->blk_dva[i]))
                        size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);

                ASSERT3P(vd, !=, NULL);

                metaslab_check_free_impl(vd, offset, size);
        }
        spa_config_exit(spa, SCL_VDEV, FTAG);
}