Merge commit '7e3488dc6cdcb0c04e1ce167a1a3bfef83b5f2e0'

[unleashed.git] / kernel / fs / zfs / spa_checkpoint.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) 2017 by Delphix. All rights reserved.
 */

/*
 * Storage Pool Checkpoint
 *
 * A storage pool checkpoint can be thought of as a pool-wide snapshot or
 * a stable version of extreme rewind that guarantees no blocks from the
 * checkpointed state will have been overwritten. It remembers the entire
 * state of the storage pool (e.g. snapshots, dataset names, etc..) from the
 * point that it was taken and the user can rewind back to that point even if
 * they applied destructive operations on their datasets or even enabled new
 * zpool on-disk features. If a pool has a checkpoint that is no longer
 * needed, the user can discard it.
 *
 * == On disk data structures used ==
 *
 * - The pool has a new feature flag and a new entry in the MOS. The feature
 *   flag is set to active when we create the checkpoint and remains active
 *   until the checkpoint is fully discarded. The entry in the MOS config
 *   (DMU_POOL_ZPOOL_CHECKPOINT) is populated with the uberblock that
 *   references the state of the pool when we take the checkpoint. The entry
 *   remains populated until we start discarding the checkpoint or we rewind
 *   back to it.
 *
 * - Each vdev contains a vdev-wide space map while the pool has a checkpoint,
 *   which persists until the checkpoint is fully discarded. The space map
 *   contains entries that have been freed in the current state of the pool
 *   but we want to keep around in case we decide to rewind to the checkpoint.
 *   [see vdev_checkpoint_sm]
 *
 * - Each metaslab's ms_sm space map behaves the same as without the
 *   checkpoint, with the only exception being the scenario when we free
 *   blocks that belong to the checkpoint. In this case, these blocks remain
 *   ALLOCATED in the metaslab's space map and they are added as FREE in the
 *   vdev's checkpoint space map.
 *
 * - Each uberblock has a field (ub_checkpoint_txg) which holds the txg that
 *   the uberblock was checkpointed. For normal uberblocks this field is 0.
 *
 * == Overview of operations ==
 *
 * - To create a checkpoint, we first wait for the current TXG to be synced,
 *   so we can use the most recently synced uberblock (spa_ubsync) as the
 *   checkpointed uberblock. Then we use an early synctask to place that
 *   uberblock in MOS config, increment the feature flag for the checkpoint
 *   (marking it active), and setting spa_checkpoint_txg (see its use below)
 *   to the TXG of the checkpointed uberblock. We use an early synctask for
 *   the aforementioned operations to ensure that no blocks were dirtied
 *   between the current TXG and the TXG of the checkpointed uberblock
 *   (e.g the previous txg).
 *
 * - When a checkpoint exists, we need to ensure that the blocks that
 *   belong to the checkpoint are freed but never reused. This means that
 *   these blocks should never end up in the ms_allocatable or the ms_freeing
 *   trees of a metaslab. Therefore, whenever there is a checkpoint the new
 *   ms_checkpointing tree is used in addition to the aforementioned ones.
 *
 *   Whenever a block is freed and we find out that it is referenced by the
 *   checkpoint (we find out by comparing its birth to spa_checkpoint_txg),
 *   we place it in the ms_checkpointing tree instead of the ms_freeingtree.
 *   This way, we divide the blocks that are being freed into checkpointed
 *   and not-checkpointed blocks.
 *
 *   In order to persist these frees, we write the extents from the
 *   ms_freeingtree to the ms_sm as usual, and the extents from the
 *   ms_checkpointing tree to the vdev_checkpoint_sm. This way, these
 *   checkpointed extents will remain allocated in the metaslab's ms_sm space
 *   map, and therefore won't be reused [see metaslab_sync()]. In addition,
 *   when we discard the checkpoint, we can find the entries that have
 *   actually been freed in vdev_checkpoint_sm.
 *   [see spa_checkpoint_discard_thread_sync()]
 *
 * - To discard the checkpoint we use an early synctask to delete the
 *   checkpointed uberblock from the MOS config, set spa_checkpoint_txg to 0,
 *   and wakeup the discarding zthr thread (an open-context async thread).
 *   We use an early synctask to ensure that the operation happens before any
 *   new data end up in the checkpoint's data structures.
 *
 *   Once the synctask is done and the discarding zthr is awake, we discard
 *   the checkpointed data over multiple TXGs by having the zthr prefetching
 *   entries from vdev_checkpoint_sm and then starting a synctask that places
 *   them as free blocks in to their respective ms_allocatable and ms_sm
 *   structures.
 *   [see spa_checkpoint_discard_thread()]
 *
 *   When there are no entries left in the vdev_checkpoint_sm of all
 *   top-level vdevs, a final synctask runs that decrements the feature flag.
 *
 * - To rewind to the checkpoint, we first use the current uberblock and
 *   open the MOS so we can access the checkpointed uberblock from the MOS
 *   config. After we retrieve the checkpointed uberblock, we use it as the
 *   current uberblock for the pool by writing it to disk with an updated
 *   TXG, opening its version of the MOS, and moving on as usual from there.
 *   [see spa_ld_checkpoint_rewind()]
 *
 *   An important note on rewinding to the checkpoint has to do with how we
 *   handle ZIL blocks. In the scenario of a rewind, we clear out any ZIL
 *   blocks that have not been claimed by the time we took the checkpoint
 *   as they should no longer be valid.
 *   [see comment in zil_claim()]
 *
 * == Miscellaneous information ==
 *
 * - In the hypothetical event that we take a checkpoint, remove a vdev,
 *   and attempt to rewind, the rewind would fail as the checkpointed
 *   uberblock would reference data in the removed device. For this reason
 *   and others of similar nature, we disallow the following operations that
 *   can change the config:
 *      vdev removal and attach/detach, mirror splitting, and pool reguid.
 *
 * - As most of the checkpoint logic is implemented in the SPA and doesn't
 *   distinguish datasets when it comes to space accounting, having a
 *   checkpoint can potentially break the boundaries set by dataset
 *   reservations.
 */

#include <sys/dmu_tx.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_synctask.h>
#include <sys/metaslab_impl.h>
#include <sys/spa.h>
#include <sys/spa_impl.h>
#include <sys/spa_checkpoint.h>
#include <sys/vdev_impl.h>
#include <sys/zap.h>
#include <sys/zfeature.h>

/*
 * The following parameter limits the amount of memory to be used for the
 * prefetching of the checkpoint space map done on each vdev while
 * discarding the checkpoint.
 *
 * The reason it exists is because top-level vdevs with long checkpoint
 * space maps can potentially take up a lot of memory depending on the
 * amount of checkpointed data that has been freed within them while
 * the pool had a checkpoint.
 */
uint64_t        zfs_spa_discard_memory_limit = 16 * 1024 * 1024;

int
spa_checkpoint_get_stats(spa_t *spa, pool_checkpoint_stat_t *pcs)
{
        if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
                return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT));

        bzero(pcs, sizeof (pool_checkpoint_stat_t));

        int error = zap_contains(spa_meta_objset(spa),
            DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT);
        ASSERT(error == 0 || error == ENOENT);

        if (error == ENOENT)
                pcs->pcs_state = CS_CHECKPOINT_DISCARDING;
        else
                pcs->pcs_state = CS_CHECKPOINT_EXISTS;

        pcs->pcs_space = spa->spa_checkpoint_info.sci_dspace;
        pcs->pcs_start_time = spa->spa_checkpoint_info.sci_timestamp;

        return (0);
}

static void
spa_checkpoint_discard_complete_sync(void *arg, dmu_tx_t *tx)
{
        spa_t *spa = arg;

        spa->spa_checkpoint_info.sci_timestamp = 0;

        spa_feature_decr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx);

        spa_history_log_internal(spa, "spa discard checkpoint", tx,
            "finished discarding checkpointed state from the pool");
}

typedef struct spa_checkpoint_discard_sync_callback_arg {
        vdev_t *sdc_vd;
        uint64_t sdc_txg;
        uint64_t sdc_entry_limit;
} spa_checkpoint_discard_sync_callback_arg_t;

static int
spa_checkpoint_discard_sync_callback(space_map_entry_t *sme, void *arg)
{
        spa_checkpoint_discard_sync_callback_arg_t *sdc = arg;
        vdev_t *vd = sdc->sdc_vd;
        metaslab_t *ms = vd->vdev_ms[sme->sme_offset >> vd->vdev_ms_shift];
        uint64_t end = sme->sme_offset + sme->sme_run;

        if (sdc->sdc_entry_limit == 0)
                return (EINTR);

        /*
         * Since the space map is not condensed, we know that
         * none of its entries is crossing the boundaries of
         * its respective metaslab.
         *
         * That said, there is no fundamental requirement that
         * the checkpoint's space map entries should not cross
         * metaslab boundaries. So if needed we could add code
         * that handles metaslab-crossing segments in the future.
         */
        VERIFY3U(sme->sme_type, ==, SM_FREE);
        VERIFY3U(sme->sme_offset, >=, ms->ms_start);
        VERIFY3U(end, <=, ms->ms_start + ms->ms_size);

        /*
         * At this point we should not be processing any
         * other frees concurrently, so the lock is technically
         * unnecessary. We use the lock anyway though to
         * potentially save ourselves from future headaches.
         */
        mutex_enter(&ms->ms_lock);
        if (range_tree_is_empty(ms->ms_freeing))
                vdev_dirty(vd, VDD_METASLAB, ms, sdc->sdc_txg);
        range_tree_add(ms->ms_freeing, sme->sme_offset, sme->sme_run);
        mutex_exit(&ms->ms_lock);

        ASSERT3U(vd->vdev_spa->spa_checkpoint_info.sci_dspace, >=,
            sme->sme_run);
        ASSERT3U(vd->vdev_stat.vs_checkpoint_space, >=, sme->sme_run);

        vd->vdev_spa->spa_checkpoint_info.sci_dspace -= sme->sme_run;
        vd->vdev_stat.vs_checkpoint_space -= sme->sme_run;
        sdc->sdc_entry_limit--;

        return (0);
}

static void
spa_checkpoint_accounting_verify(spa_t *spa)
{
        vdev_t *rvd = spa->spa_root_vdev;
        uint64_t ckpoint_sm_space_sum = 0;
        uint64_t vs_ckpoint_space_sum = 0;

        for (uint64_t c = 0; c < rvd->vdev_children; c++) {
                vdev_t *vd = rvd->vdev_child[c];

                if (vd->vdev_checkpoint_sm != NULL) {
                        ckpoint_sm_space_sum +=
                            -vd->vdev_checkpoint_sm->sm_alloc;
                        vs_ckpoint_space_sum +=
                            vd->vdev_stat.vs_checkpoint_space;
                        ASSERT3U(ckpoint_sm_space_sum, ==,
                            vs_ckpoint_space_sum);
                } else {
                        ASSERT0(vd->vdev_stat.vs_checkpoint_space);
                }
        }
        ASSERT3U(spa->spa_checkpoint_info.sci_dspace, ==, ckpoint_sm_space_sum);
}

static void
spa_checkpoint_discard_thread_sync(void *arg, dmu_tx_t *tx)
{
        vdev_t *vd = arg;
        int error;

        /*
         * The space map callback is applied only to non-debug entries.
         * Because the number of debug entries is less or equal to the
         * number of non-debug entries, we want to ensure that we only
         * read what we prefetched from open-context.
         *
         * Thus, we set the maximum entries that the space map callback
         * will be applied to be half the entries that could fit in the
         * imposed memory limit.
         *
         * Note that since this is a conservative estimate we also
         * assume the worst case scenario in our computation where each
         * entry is two-word.
         */
        uint64_t max_entry_limit =
            (zfs_spa_discard_memory_limit / (2 * sizeof (uint64_t))) >> 1;

        /*
         * Iterate from the end of the space map towards the beginning,
         * placing its entries on ms_freeing and removing them from the
         * space map. The iteration stops if one of the following
         * conditions is true:
         *
         * 1] We reached the beginning of the space map. At this point
         *    the space map should be completely empty and
         *    space_map_incremental_destroy should have returned 0.
         *    The next step would be to free and close the space map
         *    and remove its entry from its vdev's top zap. This allows
         *    spa_checkpoint_discard_thread() to move on to the next vdev.
         *
         * 2] We reached the memory limit (amount of memory used to hold
         *    space map entries in memory) and space_map_incremental_destroy
         *    returned EINTR. This means that there are entries remaining
         *    in the space map that will be cleared in a future invocation
         *    of this function by spa_checkpoint_discard_thread().
         */
        spa_checkpoint_discard_sync_callback_arg_t sdc;
        sdc.sdc_vd = vd;
        sdc.sdc_txg = tx->tx_txg;
        sdc.sdc_entry_limit = max_entry_limit;

        uint64_t words_before =
            space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t);

        error = space_map_incremental_destroy(vd->vdev_checkpoint_sm,
            spa_checkpoint_discard_sync_callback, &sdc, tx);

        uint64_t words_after =
            space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t);

#ifdef DEBUG
        spa_checkpoint_accounting_verify(vd->vdev_spa);
#endif

        zfs_dbgmsg("discarding checkpoint: txg %llu, vdev id %d, "
            "deleted %llu words - %llu words are left",
            tx->tx_txg, vd->vdev_id, (words_before - words_after),
            words_after);

        if (error != EINTR) {
                if (error != 0) {
                        zfs_panic_recover("zfs: error %d was returned "
                            "while incrementally destroying the checkpoint "
                            "space map of vdev %llu\n",
                            error, vd->vdev_id);
                }
                ASSERT0(words_after);
                ASSERT0(vd->vdev_checkpoint_sm->sm_alloc);
                ASSERT0(space_map_length(vd->vdev_checkpoint_sm));

                space_map_free(vd->vdev_checkpoint_sm, tx);
                space_map_close(vd->vdev_checkpoint_sm);
                vd->vdev_checkpoint_sm = NULL;

                VERIFY0(zap_remove(spa_meta_objset(vd->vdev_spa),
                    vd->vdev_top_zap, VDEV_TOP_ZAP_POOL_CHECKPOINT_SM, tx));
        }
}

static boolean_t
spa_checkpoint_discard_is_done(spa_t *spa)
{
        vdev_t *rvd = spa->spa_root_vdev;

        ASSERT(!spa_has_checkpoint(spa));
        ASSERT(spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT));

        for (uint64_t c = 0; c < rvd->vdev_children; c++) {
                if (rvd->vdev_child[c]->vdev_checkpoint_sm != NULL)
                        return (B_FALSE);
                ASSERT0(rvd->vdev_child[c]->vdev_stat.vs_checkpoint_space);
        }

        return (B_TRUE);
}

/* ARGSUSED */
boolean_t
spa_checkpoint_discard_thread_check(void *arg, zthr_t *zthr)
{
        spa_t *spa = arg;

        if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
                return (B_FALSE);

        if (spa_has_checkpoint(spa))
                return (B_FALSE);

        return (B_TRUE);
}

int
spa_checkpoint_discard_thread(void *arg, zthr_t *zthr)
{
        spa_t *spa = arg;
        vdev_t *rvd = spa->spa_root_vdev;

        for (uint64_t c = 0; c < rvd->vdev_children; c++) {
                vdev_t *vd = rvd->vdev_child[c];

                while (vd->vdev_checkpoint_sm != NULL) {
                        space_map_t *checkpoint_sm = vd->vdev_checkpoint_sm;
                        int numbufs;
                        dmu_buf_t **dbp;

                        if (zthr_iscancelled(zthr))
                                return (0);

                        ASSERT3P(vd->vdev_ops, !=, &vdev_indirect_ops);

                        uint64_t size = MIN(space_map_length(checkpoint_sm),
                            zfs_spa_discard_memory_limit);
                        uint64_t offset =
                            space_map_length(checkpoint_sm) - size;

                        /*
                         * Ensure that the part of the space map that will
                         * be destroyed by the synctask, is prefetched in
                         * memory before the synctask runs.
                         */
                        int error = dmu_buf_hold_array_by_bonus(
                            checkpoint_sm->sm_dbuf, offset, size,
                            B_TRUE, FTAG, &numbufs, &dbp);
                        if (error != 0) {
                                zfs_panic_recover("zfs: error %d was returned "
                                    "while prefetching checkpoint space map "
                                    "entries of vdev %llu\n",
                                    error, vd->vdev_id);
                        }

                        VERIFY0(dsl_sync_task(spa->spa_name, NULL,
                            spa_checkpoint_discard_thread_sync, vd,
                            0, ZFS_SPACE_CHECK_NONE));

                        dmu_buf_rele_array(dbp, numbufs, FTAG);
                }
        }

        VERIFY(spa_checkpoint_discard_is_done(spa));
        VERIFY0(spa->spa_checkpoint_info.sci_dspace);
        VERIFY0(dsl_sync_task(spa->spa_name, NULL,
            spa_checkpoint_discard_complete_sync, spa,
            0, ZFS_SPACE_CHECK_NONE));

        return (0);
}


/* ARGSUSED */
static int
spa_checkpoint_check(void *arg, dmu_tx_t *tx)
{
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;

        if (!spa_feature_is_enabled(spa, SPA_FEATURE_POOL_CHECKPOINT))
                return (SET_ERROR(ENOTSUP));

        if (!spa_top_vdevs_spacemap_addressable(spa))
                return (SET_ERROR(ZFS_ERR_VDEV_TOO_BIG));

        if (spa->spa_vdev_removal != NULL)
                return (SET_ERROR(ZFS_ERR_DEVRM_IN_PROGRESS));

        if (spa->spa_checkpoint_txg != 0)
                return (SET_ERROR(ZFS_ERR_CHECKPOINT_EXISTS));

        if (spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
                return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT));

        return (0);
}

/* ARGSUSED */
static void
spa_checkpoint_sync(void *arg, dmu_tx_t *tx)
{
        dsl_pool_t *dp = dmu_tx_pool(tx);
        spa_t *spa = dp->dp_spa;
        uberblock_t checkpoint = spa->spa_ubsync;

        /*
         * At this point, there should not be a checkpoint in the MOS.
         */
        ASSERT3U(zap_contains(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_ZPOOL_CHECKPOINT), ==, ENOENT);

        ASSERT0(spa->spa_checkpoint_info.sci_timestamp);
        ASSERT0(spa->spa_checkpoint_info.sci_dspace);

        /*
         * Since the checkpointed uberblock is the one that just got synced
         * (we use spa_ubsync), its txg must be equal to the txg number of
         * the txg we are syncing, minus 1.
         */
        ASSERT3U(checkpoint.ub_txg, ==, spa->spa_syncing_txg - 1);

        /*
         * Once the checkpoint is in place, we need to ensure that none of
         * its blocks will be marked for reuse after it has been freed.
         * When there is a checkpoint and a block is freed, we compare its
         * birth txg to the txg of the checkpointed uberblock to see if the
         * block is part of the checkpoint or not. Therefore, we have to set
         * spa_checkpoint_txg before any frees happen in this txg (which is
         * why this is done as an early_synctask as explained in the comment
         * in spa_checkpoint()).
         */
        spa->spa_checkpoint_txg = checkpoint.ub_txg;
        spa->spa_checkpoint_info.sci_timestamp = checkpoint.ub_timestamp;

        checkpoint.ub_checkpoint_txg = checkpoint.ub_txg;
        VERIFY0(zap_add(spa->spa_dsl_pool->dp_meta_objset,
            DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT,
            sizeof (uint64_t), sizeof (uberblock_t) / sizeof (uint64_t),
            &checkpoint, tx));

        /*
         * Increment the feature refcount and thus activate the feature.
         * Note that the feature will be deactivated when we've
         * completely discarded all checkpointed state (both vdev
         * space maps and uberblock).
         */
        spa_feature_incr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx);

        spa_history_log_internal(spa, "spa checkpoint", tx,
            "checkpointed uberblock txg=%llu", checkpoint.ub_txg);
}

/*
 * Create a checkpoint for the pool.
 */
int
spa_checkpoint(const char *pool)
{
        int error;
        spa_t *spa;

        error = spa_open(pool, &spa, FTAG);
        if (error != 0)
                return (error);

        mutex_enter(&spa->spa_vdev_top_lock);

        /*
         * Wait for current syncing txg to finish so the latest synced
         * uberblock (spa_ubsync) has all the changes that we expect
         * to see if we were to revert later to the checkpoint. In other
         * words we want the checkpointed uberblock to include/reference
         * all the changes that were pending at the time that we issued
         * the checkpoint command.
         */
        txg_wait_synced(spa_get_dsl(spa), 0);

        /*
         * As the checkpointed uberblock references blocks from the previous
         * txg (spa_ubsync) we want to ensure that are not freeing any of
         * these blocks in the same txg that the following synctask will
         * run. Thus, we run it as an early synctask, so the dirty changes
         * that are synced to disk afterwards during zios and other synctasks
         * do not reuse checkpointed blocks.
         */
        error = dsl_early_sync_task(pool, spa_checkpoint_check,
            spa_checkpoint_sync, NULL, 0, ZFS_SPACE_CHECK_NORMAL);

        mutex_exit(&spa->spa_vdev_top_lock);

        spa_close(spa, FTAG);
        return (error);
}

/* ARGSUSED */
static int
spa_checkpoint_discard_check(void *arg, dmu_tx_t *tx)
{
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;

        if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
                return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT));

        if (spa->spa_checkpoint_txg == 0)
                return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT));

        VERIFY0(zap_contains(spa_meta_objset(spa),
            DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT));

        return (0);
}

/* ARGSUSED */
static void
spa_checkpoint_discard_sync(void *arg, dmu_tx_t *tx)
{
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;

        VERIFY0(zap_remove(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_ZPOOL_CHECKPOINT, tx));

        spa->spa_checkpoint_txg = 0;

        zthr_wakeup(spa->spa_checkpoint_discard_zthr);

        spa_history_log_internal(spa, "spa discard checkpoint", tx,
            "started discarding checkpointed state from the pool");
}

/*
 * Discard the checkpoint from a pool.
 */
int
spa_checkpoint_discard(const char *pool)
{
        /*
         * Similarly to spa_checkpoint(), we want our synctask to run
         * before any pending dirty data are written to disk so they
         * won't end up in the checkpoint's data structures (e.g.
         * ms_checkpointing and vdev_checkpoint_sm) and re-create any
         * space maps that the discarding open-context thread has
         * deleted.
         * [see spa_discard_checkpoint_sync and spa_discard_checkpoint_thread]
         */
        return (dsl_early_sync_task(pool, spa_checkpoint_discard_check,
            spa_checkpoint_discard_sync, NULL, 0,
            ZFS_SPACE_CHECK_DISCARD_CHECKPOINT));
}