4976 zfs should only avoid writing to a failing non-redundant top-level vdev

[unleashed.git] / usr / src / uts / common / 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, 2014 by Delphix. All rights reserved.
 * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
 */

#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>

/*
 * Allow allocations to switch to gang blocks quickly. We do this to
 * avoid having to load lots of space_maps in a given txg. There are,
 * however, some cases where we want to avoid "fast" ganging and instead
 * we want to do an exhaustive search of all metaslabs on this device.
 * Currently we don't allow any gang, slog, or dump device related allocations
 * to "fast" gang.
 */
#define CAN_FASTGANG(flags) \
        (!((flags) & (METASLAB_GANG_CHILD | METASLAB_GANG_HEADER | \
        METASLAB_GANG_AVOID)))

#define METASLAB_WEIGHT_PRIMARY         (1ULL << 63)
#define METASLAB_WEIGHT_SECONDARY       (1ULL << 62)
#define METASLAB_ACTIVE_MASK            \
        (METASLAB_WEIGHT_PRIMARY | METASLAB_WEIGHT_SECONDARY)

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

/*
 * 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_INITIAL_BLOCKSIZE), 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_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;

static uint64_t metaslab_fragmentation(metaslab_t *);

/*
 * ==========================================================================
 * 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;

        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);

        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 (tvd->vdev_ishole || 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 (tvd->vdev_ishole || 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++) {
                vdev_t *tvd = rvd->vdev_child[c];
                metaslab_group_t *mg = tvd->vdev_mg;

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

                space += tvd->vdev_max_asize - tvd->vdev_asize;
        }
        spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
        return (space);
}

/*
 * ==========================================================================
 * Metaslab groups
 * ==========================================================================
 */
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);
}

/*
 * 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. 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;

        ASSERT(vd == vd->vdev_top);

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

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

        /*
         * 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_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(&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_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);
        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;

        ASSERT(spa_config_held(mc->mc_spa, SCL_ALLOC, RW_WRITER));

        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;
}

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

        ASSERT(spa_config_held(mc->mc_spa, SCL_ALLOC, RW_WRITER));

        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;
        }

        taskq_wait(mg->mg_taskq);
        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;
}

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.
 */
static boolean_t
metaslab_group_allocatable(metaslab_group_t *mg)
{
        vdev_t *vd = mg->mg_vd;
        spa_t *spa = vd->vdev_spa;
        metaslab_class_t *mc = mg->mg_class;

        /*
         * We use two key metrics to determine if a metaslab group is
         * considered allocatable -- free space and fragmentation. If
         * the free space is greater than the free space threshold and
         * the fragmentation is less than the fragmentation threshold then
         * consider the group allocatable. There are two case when we will
         * not consider these key metrics. The first is if the group is
         * associated with a slog device and the second is if all groups
         * in this metaslab class have already been consider ineligible
         * for allocations.
         */
        return ((mg->mg_free_capacity > zfs_mg_noalloc_threshold &&
            (mg->mg_fragmentation == ZFS_FRAG_INVALID ||
            mg->mg_fragmentation <= zfs_mg_fragmentation_threshold)) ||
            mc != spa_normal_class(spa) || mc->mc_alloc_groups == 0);
}

/*
 * ==========================================================================
 * 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_tree == NULL);

        avl_create(&msp->ms_size_tree, 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_tree, ==, rt);
        ASSERT0(avl_numnodes(&msp->ms_size_tree));

        avl_destroy(&msp->ms_size_tree);
}

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_tree, ==, rt);
        VERIFY(!msp->ms_condensing);
        avl_add(&msp->ms_size_tree, 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_tree, ==, rt);
        VERIFY(!msp->ms_condensing);
        avl_remove(&msp->ms_size_tree, rs);
}

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

        ASSERT3P(rt->rt_arg, ==, msp);
        ASSERT3P(msp->ms_tree, ==, 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_size_tree, 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
};

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

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

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

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

uint64_t
metaslab_block_alloc(metaslab_t *msp, uint64_t size)
{
        uint64_t start;
        range_tree_t *rt = msp->ms_tree;

        VERIFY(!msp->ms_condensing);

        start = msp->ms_ops->msop_alloc(msp, size);
        if (start != -1ULL) {
                vdev_t *vd = msp->ms_group->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);
        }
        return (start);
}

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

/*
 * 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, rsearch;
        avl_index_t where;

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

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

        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_tree->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_tree;
        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_size_tree));

        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_size_tree;
                *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_tree;
        avl_tree_t *t = &msp->ms_size_tree;
        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_size_tree);
                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_tree->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_size_tree));

        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_size_tree;

                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;

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

        msp->ms_loading = B_TRUE;

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

        msp->ms_loaded = (error == 0);
        msp->ms_loading = B_FALSE;

        if (msp->ms_loaded) {
                for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                        range_tree_walk(msp->ms_defertree[t],
                            range_tree_remove, msp->ms_tree);
                }
        }
        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_tree, NULL, NULL);
        msp->ms_loaded = B_FALSE;
        msp->ms_weight &= ~METASLAB_ACTIVE_MASK;
}

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

        msp = kmem_zalloc(sizeof (metaslab_t), KM_SLEEP);
        mutex_init(&msp->ms_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&msp->ms_load_cv, NULL, CV_DEFAULT, NULL);
        msp->ms_id = id;
        msp->ms_start = id << vd->vdev_ms_shift;
        msp->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) {
                VERIFY0(space_map_open(&msp->ms_sm, mos, object, msp->ms_start,
                    msp->ms_size, vd->vdev_ashift, &msp->ms_lock));
                ASSERT(msp->ms_sm != NULL);
        }

        /*
         * We create the main range tree here, but we don't create the
         * alloctree and freetree 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.
         */
        msp->ms_tree = range_tree_create(&metaslab_rt_ops, msp, &msp->ms_lock);
        metaslab_group_add(mg, msp);

        msp->ms_fragmentation = metaslab_fragmentation(msp);
        msp->ms_ops = mg->mg_class->mc_ops;

        /*
         * 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.
         */
        if (txg <= TXG_INITIAL)
                metaslab_sync_done(msp, 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 && msp->ms_sm != NULL) {
                mutex_enter(&msp->ms_lock);
                VERIFY0(metaslab_load(msp));
                mutex_exit(&msp->ms_lock);
        }

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

        return (msp);
}

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_tree);

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

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

        ASSERT0(msp->ms_deferspace);

        mutex_exit(&msp->ms_lock);
        cv_destroy(&msp->ms_load_cv);
        mutex_destroy(&msp->ms_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 uint64_t
metaslab_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)
                return (ZFS_FRAG_INVALID);

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

        /*
         * 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;

                msp->ms_condense_wanted = B_TRUE;
                vdev_dirty(vd, VDD_METASLAB, msp, txg + 1);
                spa_dbgmsg(spa, "txg %llu, requesting force condense: "
                    "msp %p, vd %p", txg, msp, vd);
                return (ZFS_FRAG_INVALID);
        }

        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);
        return (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_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));

        /*
         * This vdev is in the process of being removed so there is nothing
         * for us to do here.
         */
        if (vd->vdev_removing) {
                ASSERT0(space_map_allocated(msp->ms_sm));
                ASSERT0(vd->vdev_ms_shift);
                return (0);
        }

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

        msp->ms_fragmentation = metaslab_fragmentation(msp);
        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);
        }

        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);
                        }
                }

                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 size)
{
        /*
         * 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_space(msp->ms_tree) == 0);
        metaslab_group_sort(msp->ms_group, msp, MIN(msp->ms_weight, size));
        ASSERT((msp->ms_weight & METASLAB_ACTIVE_MASK) == 0);
}

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);

        /*
         * Set the ms_access_txg value so that we don't unload it right away.
         */
        msp->ms_access_txg = spa_syncing_txg(spa) + metaslab_unload_delay + 1;
        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
         */
        msp = avl_first(t);
        while (msp != NULL) {
                metaslab_t *msp_next = AVL_NEXT(t, msp);

                /*
                 * 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) {
                        msp = msp_next;
                        continue;
                }

                /*
                 * We must drop the metaslab group lock here to preserve
                 * lock ordering with the ms_lock (when grabbing both
                 * the mg_lock and the ms_lock, the ms_lock must be taken
                 * first).  As a result, it is possible that the ordering
                 * of the metaslabs within the avl tree may change before
                 * we reacquire the lock. The metaslab cannot be removed from
                 * the tree while we're in syncing context so it is safe to
                 * drop the mg_lock here. If the metaslabs are reordered
                 * nothing will break -- we just may end up loading a
                 * less than optimal one.
                 */
                mutex_exit(&mg->mg_lock);
                VERIFY(taskq_dispatch(mg->mg_taskq, metaslab_preload,
                    msp, TQ_SLEEP) != NULL);
                mutex_enter(&mg->mg_lock);
                msp = msp_next;
        }
        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 = 1.1.MB).
 *
 * 3. The on-disk size of the space map should actually decrease.
 *
 * Checking the first condition is tricky since we don't want to walk
 * the entire AVL tree calculating the estimated on-disk size. Instead we
 * use the size-ordered range tree in the metaslab and calculate the
 * size required to write out the largest segment in our free tree. If the
 * size required to represent that segment on disk is larger than the space
 * map object then we avoid condensing this map.
 *
 * To determine the second criterion we use a best-case estimate and assume
 * each segment can be represented on-disk as a single 64-bit entry. We refer
 * to this best-case estimate as the space map's minimal form.
 *
 * 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;
        range_seg_t *rs;
        uint64_t size, entries, segsz, object_size, optimal_size, record_size;
        dmu_object_info_t doi;
        uint64_t vdev_blocksize = 1 << msp->ms_group->mg_vd->vdev_ashift;

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

        /*
         * Use the ms_size_tree range tree, which is ordered by size, to
         * obtain the largest segment in the free tree. We always condense
         * metaslabs that are empty and metaslabs for which a condense
         * request has been made.
         */
        rs = avl_last(&msp->ms_size_tree);
        if (rs == NULL || msp->ms_condense_wanted)
                return (B_TRUE);

        /*
         * Calculate the number of 64-bit entries this segment would
         * require when written to disk. If this single segment would be
         * larger on-disk than the entire current on-disk structure, then
         * clearly condensing will increase the on-disk structure size.
         */
        size = (rs->rs_end - rs->rs_start) >> sm->sm_shift;
        entries = size / (MIN(size, SM_RUN_MAX));
        segsz = entries * sizeof (uint64_t);

        optimal_size = sizeof (uint64_t) * avl_numnodes(&msp->ms_tree->rt_root);
        object_size = space_map_length(msp->ms_sm);

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

        return (segsz <= object_size &&
            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)
{
        spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
        range_tree_t *freetree = msp->ms_freetree[txg & TXG_MASK];
        range_tree_t *condense_tree;
        space_map_t *sm = msp->ms_sm;

        ASSERT(MUTEX_HELD(&msp->ms_lock));
        ASSERT3U(spa_sync_pass(spa), ==, 1);
        ASSERT(msp->ms_loaded);


        spa_dbgmsg(spa, "condensing: txg %llu, msp[%llu] %p, "
            "smp size %llu, segments %lu, forcing condense=%s", txg,
            msp->ms_id, msp, space_map_length(msp->ms_sm),
            avl_numnodes(&msp->ms_tree->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, &msp->ms_lock);
        range_tree_add(condense_tree, msp->ms_start, msp->ms_size);

        /*
         * Remove what's been freed in this txg from the condense_tree.
         * Since we're in sync_pass 1, we know that all the frees from
         * this txg are in the freetree.
         */
        range_tree_walk(freetree, range_tree_remove, condense_tree);

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

        for (int t = 1; t < TXG_CONCURRENT_STATES; t++) {
                range_tree_walk(msp->ms_alloctree[(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 the ms_tree 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, tx);
        mutex_enter(&msp->ms_lock);

        /*
         * 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, tx);
        range_tree_vacate(condense_tree, NULL, NULL);
        range_tree_destroy(condense_tree);

        space_map_write(sm, msp->ms_tree, SM_FREE, tx);
        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_alloctree[txg & TXG_MASK];
        range_tree_t **freetree = &msp->ms_freetree[txg & TXG_MASK];
        range_tree_t **freed_tree =
            &msp->ms_freetree[TXG_CLEAN(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 (*freetree == NULL) {
                ASSERT3P(alloctree, ==, NULL);
                return;
        }

        ASSERT3P(alloctree, !=, NULL);
        ASSERT3P(*freetree, !=, NULL);
        ASSERT3P(*freed_tree, !=, 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 we need to let it through.
         */
        if (range_tree_space(alloctree) == 0 &&
            range_tree_space(*freetree) == 0 &&
            !msp->ms_condense_wanted)
                return;

        /*
         * The only state that can actually be changing concurrently with
         * metaslab_sync() is the metaslab's ms_tree.  No other thread can
         * be modifying this txg's alloctree, freetree, freed_tree, or
         * space_map_phys_t. Therefore, we only hold ms_lock to satify
         * space_map ASSERTs. We drop it whenever we call into the DMU,
         * because the DMU can call down to us (e.g. via zio_free()) at
         * any time.
         */

        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, tx);
                VERIFY3U(new_object, !=, 0);

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

        mutex_enter(&msp->ms_lock);

        if (msp->ms_loaded && spa_sync_pass(spa) == 1 &&
            metaslab_should_condense(msp)) {
                metaslab_condense(msp, txg, tx);
        } else {
                space_map_write(msp->ms_sm, alloctree, SM_ALLOC, tx);
                space_map_write(msp->ms_sm, *freetree, SM_FREE, tx);
        }

        metaslab_group_histogram_verify(mg);
        metaslab_class_histogram_verify(mg->mg_class);
        metaslab_group_histogram_remove(mg, msp);
        if (msp->ms_loaded) {
                /*
                 * When the space map is loaded, we have an accruate
                 * 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_tree, tx);
        } else {
                /*
                 * Since the space map is not loaded we simply update the
                 * exisiting histogram with what was freed in this txg. This
                 * means that the on-disk histogram may not have an accurate
                 * view of the free space but it's close enough to allow
                 * us to make allocation decisions.
                 */
                space_map_histogram_add(msp->ms_sm, *freetree, 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 freetree and
         * freed_tree. 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(freetree, freed_tree);
        } else {
                range_tree_vacate(*freetree, range_tree_add, *freed_tree);
        }
        range_tree_vacate(alloctree, NULL, NULL);

        ASSERT0(range_tree_space(msp->ms_alloctree[txg & TXG_MASK]));
        ASSERT0(range_tree_space(msp->ms_freetree[txg & TXG_MASK]));

        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);
        }
        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;
        range_tree_t **freed_tree;
        range_tree_t **defer_tree;
        int64_t alloc_delta, defer_delta;

        ASSERT(!vd->vdev_ishole);

        mutex_enter(&msp->ms_lock);

        /*
         * If this metaslab is just becoming available, initialize its
         * alloctrees, freetrees, and defertree and add its capacity to
         * the vdev.
         */
        if (msp->ms_freetree[TXG_CLEAN(txg) & TXG_MASK] == NULL) {
                for (int t = 0; t < TXG_SIZE; t++) {
                        ASSERT(msp->ms_alloctree[t] == NULL);
                        ASSERT(msp->ms_freetree[t] == NULL);

                        msp->ms_alloctree[t] = range_tree_create(NULL, msp,
                            &msp->ms_lock);
                        msp->ms_freetree[t] = range_tree_create(NULL, msp,
                            &msp->ms_lock);
                }

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

                        msp->ms_defertree[t] = range_tree_create(NULL, msp,
                            &msp->ms_lock);
                }

                vdev_space_update(vd, 0, 0, msp->ms_size);
        }

        freed_tree = &msp->ms_freetree[TXG_CLEAN(txg) & TXG_MASK];
        defer_tree = &msp->ms_defertree[txg % TXG_DEFER_SIZE];

        alloc_delta = space_map_alloc_delta(msp->ms_sm);
        defer_delta = range_tree_space(*freed_tree) -
            range_tree_space(*defer_tree);

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

        ASSERT0(range_tree_space(msp->ms_alloctree[txg & TXG_MASK]));
        ASSERT0(range_tree_space(msp->ms_freetree[txg & TXG_MASK]));

        /*
         * 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_tree);
        range_tree_swap(freed_tree, defer_tree);

        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);
        }

        if (msp->ms_loaded && msp->ms_access_txg < txg) {
                for (int t = 1; t < TXG_CONCURRENT_STATES; t++) {
                        VERIFY0(range_tree_space(
                            msp->ms_alloctree[(txg + t) & TXG_MASK]));
                }

                if (!metaslab_debug_unload)
                        metaslab_unload(msp);
        }

        metaslab_group_sort(mg, msp, metaslab_weight(msp));
        mutex_exit(&msp->ms_lock);
}

void
metaslab_sync_reassess(metaslab_group_t *mg)
{
        metaslab_group_alloc_update(mg);
        mg->mg_fragmentation = metaslab_group_fragmentation(mg);

        /*
         * Preload the next potential metaslabs
         */
        metaslab_group_preload(mg);
}

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);
}

static uint64_t
metaslab_group_alloc(metaslab_group_t *mg, uint64_t psize, uint64_t asize,
    uint64_t txg, uint64_t min_distance, dva_t *dva, int d)
{
        spa_t *spa = mg->mg_vd->vdev_spa;
        metaslab_t *msp = NULL;
        uint64_t offset = -1ULL;
        avl_tree_t *t = &mg->mg_metaslab_tree;
        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;
                }
        }

        for (;;) {
                boolean_t was_active;

                mutex_enter(&mg->mg_lock);
                for (msp = avl_first(t); msp; msp = AVL_NEXT(t, msp)) {
                        if (msp->ms_weight < asize) {
                                spa_dbgmsg(spa, "%s: failed to meet weight "
                                    "requirement: vdev %llu, txg %llu, mg %p, "
                                    "msp %p, psize %llu, asize %llu, "
                                    "weight %llu", spa_name(spa),
                                    mg->mg_vd->vdev_id, txg,
                                    mg, msp, psize, asize, msp->ms_weight);
                                mutex_exit(&mg->mg_lock);
                                return (-1ULL);
                        }

                        /*
                         * 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)
                        return (-1ULL);

                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.
                 */
                if (msp->ms_weight < asize || (was_active &&
                    !(msp->ms_weight & METASLAB_ACTIVE_MASK) &&
                    activation_weight == METASLAB_WEIGHT_PRIMARY)) {
                        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;
                }

                /*
                 * 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) {
                        mutex_exit(&msp->ms_lock);
                        continue;
                }

                if ((offset = metaslab_block_alloc(msp, asize)) != -1ULL)
                        break;

                metaslab_passivate(msp, metaslab_block_maxsize(msp));
                mutex_exit(&msp->ms_lock);
        }

        if (range_tree_space(msp->ms_alloctree[txg & TXG_MASK]) == 0)
                vdev_dirty(mg->mg_vd, VDD_METASLAB, msp, txg);

        range_tree_add(msp->ms_alloctree[txg & TXG_MASK], offset, asize);
        msp->ms_access_txg = txg + metaslab_unload_delay;

        mutex_exit(&msp->ms_lock);

        return (offset);
}

/*
 * Allocate a block for the specified i/o.
 */
static 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)
{
        metaslab_group_t *mg, *rotor;
        vdev_t *vd;
        int dshift = 3;
        int all_zero;
        int zio_lock = B_FALSE;
        boolean_t allocatable;
        uint64_t offset = -1ULL;
        uint64_t asize;
        uint64_t distance;

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

        /*
         * For testing, make some blocks above a certain size be gang blocks.
         */
        if (psize >= metaslab_gang_bang && (ddi_get_lbolt() & 3) == 0)
                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 (i.e. removed). Consult the rotor when
                 * all else fails.
                 */
                if (vd != 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:
        all_zero = B_TRUE;
        do {
                ASSERT(mg->mg_activation_count == 1);

                vd = mg->mg_vd;

                /*
                 * Don't allocate from faulted devices.
                 */
                if (zio_lock) {
                        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 or have requested
                 * an allocation for the smallest gang block size
                 * then we don't want to avoid allocating to the this
                 * metaslab group. If we're in this condition we should
                 * try to allocate from any device possible so that we
                 * don't inadvertently return ENOSPC and suspend the pool
                 * even though space is still available.
                 */
                if (allocatable && CAN_FASTGANG(flags) &&
                    psize > SPA_GANGBLOCKSIZE)
                        allocatable = metaslab_group_allocatable(mg);

                if (!allocatable)
                        goto next;

                /*
                 * Avoid writing single-copy data to a failing vdev
                 * unless the user instructs us that it is okay.
                 */
                if ((vd->vdev_stat.vs_write_errors > 0 ||
                    vd->vdev_state < VDEV_STATE_HEALTHY) &&
                    d == 0 && dshift == 3 && vd->vdev_children == 0) {
                        all_zero = B_FALSE;
                        goto next;
                }

                ASSERT(mg->mg_class == mc);

                distance = vd->vdev_asize >> dshift;
                if (distance <= (1ULL << vd->vdev_ms_shift))
                        distance = 0;
                else
                        all_zero = B_FALSE;

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

                offset = metaslab_group_alloc(mg, psize, 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 (!all_zero) {
                dshift++;
                ASSERT(dshift < 64);
                goto top;
        }

        if (!allocatable && !zio_lock) {
                dshift = 3;
                zio_lock = B_TRUE;
                goto top;
        }

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

        return (SET_ERROR(ENOSPC));
}

/*
 * Free the block represented by DVA in the context of the specified
 * transaction group.
 */
static void
metaslab_free_dva(spa_t *spa, const dva_t *dva, uint64_t txg, boolean_t now)
{
        uint64_t vdev = DVA_GET_VDEV(dva);
        uint64_t offset = DVA_GET_OFFSET(dva);
        uint64_t size = DVA_GET_ASIZE(dva);
        vdev_t *vd;
        metaslab_t *msp;

        ASSERT(DVA_IS_VALID(dva));

        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;
        }

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

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

        mutex_enter(&msp->ms_lock);

        if (now) {
                range_tree_remove(msp->ms_alloctree[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_tree) + size, <=,
                    msp->ms_size);
                VERIFY0(P2PHASE(offset, 1ULL << vd->vdev_ashift));
                VERIFY0(P2PHASE(size, 1ULL << vd->vdev_ashift));
                range_tree_add(msp->ms_tree, offset, size);
        } else {
                if (range_tree_space(msp->ms_freetree[txg & TXG_MASK]) == 0)
                        vdev_dirty(vd, VDD_METASLAB, msp, txg);
                range_tree_add(msp->ms_freetree[txg & TXG_MASK],
                    offset, size);
        }

        mutex_exit(&msp->ms_lock);
}

/*
 * 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;
        metaslab_t *msp;
        int error = 0;

        ASSERT(DVA_IS_VALID(dva));

        if ((vd = vdev_lookup_top(spa, vdev)) == NULL ||
            (offset >> vd->vdev_ms_shift) >= vd->vdev_ms_count)
                return (SET_ERROR(ENXIO));

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

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

        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_tree, 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_tree) - size, <=, msp->ms_size);
        range_tree_remove(msp->ms_tree, offset, size);

        if (spa_writeable(spa)) {       /* don't dirty if we're zdb(1M) */
                if (range_tree_space(msp->ms_alloctree[txg & TXG_MASK]) == 0)
                        vdev_dirty(vd, VDD_METASLAB, msp, txg);
                range_tree_add(msp->ms_alloctree[txg & TXG_MASK], offset, size);
        }

        mutex_exit(&msp->ms_lock);

        return (0);
}

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)
{
        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));

        for (int d = 0; d < ndvas; d++) {
                error = metaslab_alloc_dva(spa, mc, psize, dva, d, hintdva,
                    txg, flags);
                if (error != 0) {
                        for (d--; d >= 0; d--) {
                                metaslab_free_dva(spa, &dva[d], txg, B_TRUE);
                                bzero(&dva[d], sizeof (dva_t));
                        }
                        spa_config_exit(spa, SCL_ALLOC, FTAG);
                        return (error);
                }
        }
        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));

        spa_config_enter(spa, SCL_FREE, FTAG, RW_READER);

        for (int d = 0; d < ndvas; d++)
                metaslab_free_dva(spa, &dva[d], txg, now);

        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);
}

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]);
                metaslab_t *msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];

                if (msp->ms_loaded)
                        range_tree_verify(msp->ms_tree, offset, size);

                for (int j = 0; j < TXG_SIZE; j++)
                        range_tree_verify(msp->ms_freetree[j], offset, size);
                for (int j = 0; j < TXG_DEFER_SIZE; j++)
                        range_tree_verify(msp->ms_defertree[j], offset, size);
        }
        spa_config_exit(spa, SCL_VDEV, FTAG);
}