9290 device removal reduces redundancy of mirrors

[unleashed.git] / usr / src / uts / common / fs / zfs / dsl_scan.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) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2011, 2017 by Delphix. All rights reserved.
 * Copyright 2016 Gary Mills
 * Copyright (c) 2011, 2017 by Delphix. All rights reserved.
 * Copyright 2017 Joyent, Inc.
 * Copyright (c) 2017 Datto Inc.
 */

#include <sys/dsl_scan.h>
#include <sys/dsl_pool.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_prop.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_synctask.h>
#include <sys/dnode.h>
#include <sys/dmu_tx.h>
#include <sys/dmu_objset.h>
#include <sys/arc.h>
#include <sys/zap.h>
#include <sys/zio.h>
#include <sys/zfs_context.h>
#include <sys/fs/zfs.h>
#include <sys/zfs_znode.h>
#include <sys/spa_impl.h>
#include <sys/vdev_impl.h>
#include <sys/zil_impl.h>
#include <sys/zio_checksum.h>
#include <sys/ddt.h>
#include <sys/sa.h>
#include <sys/sa_impl.h>
#include <sys/zfeature.h>
#include <sys/abd.h>
#ifdef _KERNEL
#include <sys/zfs_vfsops.h>
#endif

typedef int (scan_cb_t)(dsl_pool_t *, const blkptr_t *,
    const zbookmark_phys_t *);

static scan_cb_t dsl_scan_scrub_cb;
static void dsl_scan_cancel_sync(void *, dmu_tx_t *);
static void dsl_scan_sync_state(dsl_scan_t *, dmu_tx_t *);
static boolean_t dsl_scan_restarting(dsl_scan_t *, dmu_tx_t *);

int zfs_top_maxinflight = 32;           /* maximum I/Os per top-level */
int zfs_resilver_delay = 2;             /* number of ticks to delay resilver */
int zfs_scrub_delay = 4;                /* number of ticks to delay scrub */
int zfs_scan_idle = 50;                 /* idle window in clock ticks */

int zfs_scan_min_time_ms = 1000; /* min millisecs to scrub per txg */
int zfs_free_min_time_ms = 1000; /* min millisecs to free per txg */
int zfs_obsolete_min_time_ms = 500; /* min millisecs to obsolete per txg */
int zfs_resilver_min_time_ms = 3000; /* min millisecs to resilver per txg */
boolean_t zfs_no_scrub_io = B_FALSE; /* set to disable scrub i/o */
boolean_t zfs_no_scrub_prefetch = B_FALSE; /* set to disable scrub prefetch */
enum ddt_class zfs_scrub_ddt_class_max = DDT_CLASS_DUPLICATE;
int dsl_scan_delay_completion = B_FALSE; /* set to delay scan completion */
/* max number of blocks to free in a single TXG */
uint64_t zfs_async_block_max_blocks = UINT64_MAX;

#define DSL_SCAN_IS_SCRUB_RESILVER(scn) \
        ((scn)->scn_phys.scn_func == POOL_SCAN_SCRUB || \
        (scn)->scn_phys.scn_func == POOL_SCAN_RESILVER)

extern int zfs_txg_timeout;

/*
 * Enable/disable the processing of the free_bpobj object.
 */
boolean_t zfs_free_bpobj_enabled = B_TRUE;

/* the order has to match pool_scan_type */
static scan_cb_t *scan_funcs[POOL_SCAN_FUNCS] = {
        NULL,
        dsl_scan_scrub_cb,      /* POOL_SCAN_SCRUB */
        dsl_scan_scrub_cb,      /* POOL_SCAN_RESILVER */
};

int
dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
{
        int err;
        dsl_scan_t *scn;
        spa_t *spa = dp->dp_spa;
        uint64_t f;

        scn = dp->dp_scan = kmem_zalloc(sizeof (dsl_scan_t), KM_SLEEP);
        scn->scn_dp = dp;

        /*
         * It's possible that we're resuming a scan after a reboot so
         * make sure that the scan_async_destroying flag is initialized
         * appropriately.
         */
        ASSERT(!scn->scn_async_destroying);
        scn->scn_async_destroying = spa_feature_is_active(dp->dp_spa,
            SPA_FEATURE_ASYNC_DESTROY);

        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            "scrub_func", sizeof (uint64_t), 1, &f);
        if (err == 0) {
                /*
                 * There was an old-style scrub in progress.  Restart a
                 * new-style scrub from the beginning.
                 */
                scn->scn_restart_txg = txg;
                zfs_dbgmsg("old-style scrub was in progress; "
                    "restarting new-style scrub in txg %llu",
                    scn->scn_restart_txg);

                /*
                 * Load the queue obj from the old location so that it
                 * can be freed by dsl_scan_done().
                 */
                (void) zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
                    "scrub_queue", sizeof (uint64_t), 1,
                    &scn->scn_phys.scn_queue_obj);
        } else {
                err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
                    DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
                    &scn->scn_phys);
                if (err == ENOENT)
                        return (0);
                else if (err)
                        return (err);

                if (scn->scn_phys.scn_state == DSS_SCANNING &&
                    spa_prev_software_version(dp->dp_spa) < SPA_VERSION_SCAN) {
                        /*
                         * A new-type scrub was in progress on an old
                         * pool, and the pool was accessed by old
                         * software.  Restart from the beginning, since
                         * the old software may have changed the pool in
                         * the meantime.
                         */
                        scn->scn_restart_txg = txg;
                        zfs_dbgmsg("new-style scrub was modified "
                            "by old software; restarting in txg %llu",
                            scn->scn_restart_txg);
                }
        }

        spa_scan_stat_init(spa);
        return (0);
}

void
dsl_scan_fini(dsl_pool_t *dp)
{
        if (dp->dp_scan) {
                kmem_free(dp->dp_scan, sizeof (dsl_scan_t));
                dp->dp_scan = NULL;
        }
}

/* ARGSUSED */
static int
dsl_scan_setup_check(void *arg, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;

        if (scn->scn_phys.scn_state == DSS_SCANNING)
                return (SET_ERROR(EBUSY));

        return (0);
}

static void
dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
        pool_scan_func_t *funcp = arg;
        dmu_object_type_t ot = 0;
        dsl_pool_t *dp = scn->scn_dp;
        spa_t *spa = dp->dp_spa;

        ASSERT(scn->scn_phys.scn_state != DSS_SCANNING);
        ASSERT(*funcp > POOL_SCAN_NONE && *funcp < POOL_SCAN_FUNCS);
        bzero(&scn->scn_phys, sizeof (scn->scn_phys));
        scn->scn_phys.scn_func = *funcp;
        scn->scn_phys.scn_state = DSS_SCANNING;
        scn->scn_phys.scn_min_txg = 0;
        scn->scn_phys.scn_max_txg = tx->tx_txg;
        scn->scn_phys.scn_ddt_class_max = DDT_CLASSES - 1; /* the entire DDT */
        scn->scn_phys.scn_start_time = gethrestime_sec();
        scn->scn_phys.scn_errors = 0;
        scn->scn_phys.scn_to_examine = spa->spa_root_vdev->vdev_stat.vs_alloc;
        scn->scn_restart_txg = 0;
        scn->scn_done_txg = 0;
        spa_scan_stat_init(spa);

        if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
                scn->scn_phys.scn_ddt_class_max = zfs_scrub_ddt_class_max;

                /* rewrite all disk labels */
                vdev_config_dirty(spa->spa_root_vdev);

                if (vdev_resilver_needed(spa->spa_root_vdev,
                    &scn->scn_phys.scn_min_txg, &scn->scn_phys.scn_max_txg)) {
                        spa_event_notify(spa, NULL, NULL,
                            ESC_ZFS_RESILVER_START);
                } else {
                        spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_START);
                }

                spa->spa_scrub_started = B_TRUE;
                /*
                 * If this is an incremental scrub, limit the DDT scrub phase
                 * to just the auto-ditto class (for correctness); the rest
                 * of the scrub should go faster using top-down pruning.
                 */
                if (scn->scn_phys.scn_min_txg > TXG_INITIAL)
                        scn->scn_phys.scn_ddt_class_max = DDT_CLASS_DITTO;

        }

        /* back to the generic stuff */

        if (dp->dp_blkstats == NULL) {
                dp->dp_blkstats =
                    kmem_alloc(sizeof (zfs_all_blkstats_t), KM_SLEEP);
        }
        bzero(dp->dp_blkstats, sizeof (zfs_all_blkstats_t));

        if (spa_version(spa) < SPA_VERSION_DSL_SCRUB)
                ot = DMU_OT_ZAP_OTHER;

        scn->scn_phys.scn_queue_obj = zap_create(dp->dp_meta_objset,
            ot ? ot : DMU_OT_SCAN_QUEUE, DMU_OT_NONE, 0, tx);

        dsl_scan_sync_state(scn, tx);

        spa_history_log_internal(spa, "scan setup", tx,
            "func=%u mintxg=%llu maxtxg=%llu",
            *funcp, scn->scn_phys.scn_min_txg, scn->scn_phys.scn_max_txg);
}

/* ARGSUSED */
static void
dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
{
        static const char *old_names[] = {
                "scrub_bookmark",
                "scrub_ddt_bookmark",
                "scrub_ddt_class_max",
                "scrub_queue",
                "scrub_min_txg",
                "scrub_max_txg",
                "scrub_func",
                "scrub_errors",
                NULL
        };

        dsl_pool_t *dp = scn->scn_dp;
        spa_t *spa = dp->dp_spa;
        int i;

        /* Remove any remnants of an old-style scrub. */
        for (i = 0; old_names[i]; i++) {
                (void) zap_remove(dp->dp_meta_objset,
                    DMU_POOL_DIRECTORY_OBJECT, old_names[i], tx);
        }

        if (scn->scn_phys.scn_queue_obj != 0) {
                VERIFY(0 == dmu_object_free(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, tx));
                scn->scn_phys.scn_queue_obj = 0;
        }

        scn->scn_phys.scn_flags &= ~DSF_SCRUB_PAUSED;

        /*
         * If we were "restarted" from a stopped state, don't bother
         * with anything else.
         */
        if (scn->scn_phys.scn_state != DSS_SCANNING)
                return;

        if (complete)
                scn->scn_phys.scn_state = DSS_FINISHED;
        else
                scn->scn_phys.scn_state = DSS_CANCELED;

        if (dsl_scan_restarting(scn, tx))
                spa_history_log_internal(spa, "scan aborted, restarting", tx,
                    "errors=%llu", spa_get_errlog_size(spa));
        else if (!complete)
                spa_history_log_internal(spa, "scan cancelled", tx,
                    "errors=%llu", spa_get_errlog_size(spa));
        else
                spa_history_log_internal(spa, "scan done", tx,
                    "errors=%llu", spa_get_errlog_size(spa));

        if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
                mutex_enter(&spa->spa_scrub_lock);
                while (spa->spa_scrub_inflight > 0) {
                        cv_wait(&spa->spa_scrub_io_cv,
                            &spa->spa_scrub_lock);
                }
                mutex_exit(&spa->spa_scrub_lock);
                spa->spa_scrub_started = B_FALSE;
                spa->spa_scrub_active = B_FALSE;

                /*
                 * If the scrub/resilver completed, update all DTLs to
                 * reflect this.  Whether it succeeded or not, vacate
                 * all temporary scrub DTLs.
                 *
                 * As the scrub does not currently support traversing
                 * data that have been freed but are part of a checkpoint,
                 * we don't mark the scrub as done in the DTLs as faults
                 * may still exist in those vdevs.
                 */
                if (complete &&
                    !spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT)) {
                        vdev_dtl_reassess(spa->spa_root_vdev, tx->tx_txg,
                            scn->scn_phys.scn_max_txg, B_TRUE);

                        spa_event_notify(spa, NULL, NULL,
                            scn->scn_phys.scn_min_txg ?
                            ESC_ZFS_RESILVER_FINISH : ESC_ZFS_SCRUB_FINISH);
                } else {
                        vdev_dtl_reassess(spa->spa_root_vdev, tx->tx_txg,
                            0, B_TRUE);
                }
                spa_errlog_rotate(spa);

                /*
                 * We may have finished replacing a device.
                 * Let the async thread assess this and handle the detach.
                 */
                spa_async_request(spa, SPA_ASYNC_RESILVER_DONE);
        }

        scn->scn_phys.scn_end_time = gethrestime_sec();
}

/* ARGSUSED */
static int
dsl_scan_cancel_check(void *arg, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;

        if (scn->scn_phys.scn_state != DSS_SCANNING)
                return (SET_ERROR(ENOENT));
        return (0);
}

/* ARGSUSED */
static void
dsl_scan_cancel_sync(void *arg, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;

        dsl_scan_done(scn, B_FALSE, tx);
        dsl_scan_sync_state(scn, tx);
        spa_event_notify(scn->scn_dp->dp_spa, NULL, NULL, ESC_ZFS_SCRUB_ABORT);
}

int
dsl_scan_cancel(dsl_pool_t *dp)
{
        return (dsl_sync_task(spa_name(dp->dp_spa), dsl_scan_cancel_check,
            dsl_scan_cancel_sync, NULL, 3, ZFS_SPACE_CHECK_RESERVED));
}

boolean_t
dsl_scan_is_paused_scrub(const dsl_scan_t *scn)
{
        if (dsl_scan_scrubbing(scn->scn_dp) &&
            scn->scn_phys.scn_flags & DSF_SCRUB_PAUSED)
                return (B_TRUE);

        return (B_FALSE);
}

static int
dsl_scrub_pause_resume_check(void *arg, dmu_tx_t *tx)
{
        pool_scrub_cmd_t *cmd = arg;
        dsl_pool_t *dp = dmu_tx_pool(tx);
        dsl_scan_t *scn = dp->dp_scan;

        if (*cmd == POOL_SCRUB_PAUSE) {
                /* can't pause a scrub when there is no in-progress scrub */
                if (!dsl_scan_scrubbing(dp))
                        return (SET_ERROR(ENOENT));

                /* can't pause a paused scrub */
                if (dsl_scan_is_paused_scrub(scn))
                        return (SET_ERROR(EBUSY));
        } else if (*cmd != POOL_SCRUB_NORMAL) {
                return (SET_ERROR(ENOTSUP));
        }

        return (0);
}

static void
dsl_scrub_pause_resume_sync(void *arg, dmu_tx_t *tx)
{
        pool_scrub_cmd_t *cmd = arg;
        dsl_pool_t *dp = dmu_tx_pool(tx);
        spa_t *spa = dp->dp_spa;
        dsl_scan_t *scn = dp->dp_scan;

        if (*cmd == POOL_SCRUB_PAUSE) {
                /* can't pause a scrub when there is no in-progress scrub */
                spa->spa_scan_pass_scrub_pause = gethrestime_sec();
                scn->scn_phys.scn_flags |= DSF_SCRUB_PAUSED;
                dsl_scan_sync_state(scn, tx);
                spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_PAUSED);
        } else {
                ASSERT3U(*cmd, ==, POOL_SCRUB_NORMAL);
                if (dsl_scan_is_paused_scrub(scn)) {
                        /*
                         * We need to keep track of how much time we spend
                         * paused per pass so that we can adjust the scrub rate
                         * shown in the output of 'zpool status'
                         */
                        spa->spa_scan_pass_scrub_spent_paused +=
                            gethrestime_sec() - spa->spa_scan_pass_scrub_pause;
                        spa->spa_scan_pass_scrub_pause = 0;
                        scn->scn_phys.scn_flags &= ~DSF_SCRUB_PAUSED;
                        dsl_scan_sync_state(scn, tx);
                }
        }
}

/*
 * Set scrub pause/resume state if it makes sense to do so
 */
int
dsl_scrub_set_pause_resume(const dsl_pool_t *dp, pool_scrub_cmd_t cmd)
{
        return (dsl_sync_task(spa_name(dp->dp_spa),
            dsl_scrub_pause_resume_check, dsl_scrub_pause_resume_sync, &cmd, 3,
            ZFS_SPACE_CHECK_RESERVED));
}

boolean_t
dsl_scan_scrubbing(const dsl_pool_t *dp)
{
        dsl_scan_t *scn = dp->dp_scan;

        if (scn->scn_phys.scn_state == DSS_SCANNING &&
            scn->scn_phys.scn_func == POOL_SCAN_SCRUB)
                return (B_TRUE);

        return (B_FALSE);
}

static void dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
    dnode_phys_t *dnp, dsl_dataset_t *ds, dsl_scan_t *scn,
    dmu_objset_type_t ostype, dmu_tx_t *tx);
static void dsl_scan_visitdnode(dsl_scan_t *, dsl_dataset_t *ds,
    dmu_objset_type_t ostype,
    dnode_phys_t *dnp, uint64_t object, dmu_tx_t *tx);

void
dsl_free(dsl_pool_t *dp, uint64_t txg, const blkptr_t *bp)
{
        zio_free(dp->dp_spa, txg, bp);
}

void
dsl_free_sync(zio_t *pio, dsl_pool_t *dp, uint64_t txg, const blkptr_t *bpp)
{
        ASSERT(dsl_pool_sync_context(dp));
        zio_nowait(zio_free_sync(pio, dp->dp_spa, txg, bpp, pio->io_flags));
}

static uint64_t
dsl_scan_ds_maxtxg(dsl_dataset_t *ds)
{
        uint64_t smt = ds->ds_dir->dd_pool->dp_scan->scn_phys.scn_max_txg;
        if (ds->ds_is_snapshot)
                return (MIN(smt, dsl_dataset_phys(ds)->ds_creation_txg));
        return (smt);
}

static void
dsl_scan_sync_state(dsl_scan_t *scn, dmu_tx_t *tx)
{
        VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
            DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
            &scn->scn_phys, tx));
}

extern int zfs_vdev_async_write_active_min_dirty_percent;

static boolean_t
dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
{
        /* we never skip user/group accounting objects */
        if (zb && (int64_t)zb->zb_object < 0)
                return (B_FALSE);

        if (scn->scn_suspending)
                return (B_TRUE); /* we're already suspending */

        if (!ZB_IS_ZERO(&scn->scn_phys.scn_bookmark))
                return (B_FALSE); /* we're resuming */

        /* We only know how to resume from level-0 blocks. */
        if (zb && zb->zb_level != 0)
                return (B_FALSE);

        /*
         * We suspend if:
         *  - we have scanned for the maximum time: an entire txg
         *    timeout (default 5 sec)
         *  or
         *  - we have scanned for at least the minimum time (default 1 sec
         *    for scrub, 3 sec for resilver), and either we have sufficient
         *    dirty data that we are starting to write more quickly
         *    (default 30%), or someone is explicitly waiting for this txg
         *    to complete.
         *  or
         *  - the spa is shutting down because this pool is being exported
         *    or the machine is rebooting.
         */
        int mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
            zfs_resilver_min_time_ms : zfs_scan_min_time_ms;
        uint64_t elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
        int dirty_pct = scn->scn_dp->dp_dirty_total * 100 / zfs_dirty_data_max;
        if (elapsed_nanosecs / NANOSEC >= zfs_txg_timeout ||
            (NSEC2MSEC(elapsed_nanosecs) > mintime &&
            (txg_sync_waiting(scn->scn_dp) ||
            dirty_pct >= zfs_vdev_async_write_active_min_dirty_percent)) ||
            spa_shutting_down(scn->scn_dp->dp_spa)) {
                if (zb) {
                        dprintf("suspending at bookmark %llx/%llx/%llx/%llx\n",
                            (longlong_t)zb->zb_objset,
                            (longlong_t)zb->zb_object,
                            (longlong_t)zb->zb_level,
                            (longlong_t)zb->zb_blkid);
                        scn->scn_phys.scn_bookmark = *zb;
                }
                dprintf("suspending at DDT bookmark %llx/%llx/%llx/%llx\n",
                    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_class,
                    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_type,
                    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_checksum,
                    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_cursor);
                scn->scn_suspending = B_TRUE;
                return (B_TRUE);
        }
        return (B_FALSE);
}

typedef struct zil_scan_arg {
        dsl_pool_t      *zsa_dp;
        zil_header_t    *zsa_zh;
} zil_scan_arg_t;

/* ARGSUSED */
static int
dsl_scan_zil_block(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
{
        zil_scan_arg_t *zsa = arg;
        dsl_pool_t *dp = zsa->zsa_dp;
        dsl_scan_t *scn = dp->dp_scan;
        zil_header_t *zh = zsa->zsa_zh;
        zbookmark_phys_t zb;

        if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
                return (0);

        /*
         * One block ("stubby") can be allocated a long time ago; we
         * want to visit that one because it has been allocated
         * (on-disk) even if it hasn't been claimed (even though for
         * scrub there's nothing to do to it).
         */
        if (claim_txg == 0 && bp->blk_birth >= spa_min_claim_txg(dp->dp_spa))
                return (0);

        SET_BOOKMARK(&zb, zh->zh_log.blk_cksum.zc_word[ZIL_ZC_OBJSET],
            ZB_ZIL_OBJECT, ZB_ZIL_LEVEL, bp->blk_cksum.zc_word[ZIL_ZC_SEQ]);

        VERIFY(0 == scan_funcs[scn->scn_phys.scn_func](dp, bp, &zb));
        return (0);
}

/* ARGSUSED */
static int
dsl_scan_zil_record(zilog_t *zilog, lr_t *lrc, void *arg, uint64_t claim_txg)
{
        if (lrc->lrc_txtype == TX_WRITE) {
                zil_scan_arg_t *zsa = arg;
                dsl_pool_t *dp = zsa->zsa_dp;
                dsl_scan_t *scn = dp->dp_scan;
                zil_header_t *zh = zsa->zsa_zh;
                lr_write_t *lr = (lr_write_t *)lrc;
                blkptr_t *bp = &lr->lr_blkptr;
                zbookmark_phys_t zb;

                if (BP_IS_HOLE(bp) ||
                    bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
                        return (0);

                /*
                 * birth can be < claim_txg if this record's txg is
                 * already txg sync'ed (but this log block contains
                 * other records that are not synced)
                 */
                if (claim_txg == 0 || bp->blk_birth < claim_txg)
                        return (0);

                SET_BOOKMARK(&zb, zh->zh_log.blk_cksum.zc_word[ZIL_ZC_OBJSET],
                    lr->lr_foid, ZB_ZIL_LEVEL,
                    lr->lr_offset / BP_GET_LSIZE(bp));

                VERIFY(0 == scan_funcs[scn->scn_phys.scn_func](dp, bp, &zb));
        }
        return (0);
}

static void
dsl_scan_zil(dsl_pool_t *dp, zil_header_t *zh)
{
        uint64_t claim_txg = zh->zh_claim_txg;
        zil_scan_arg_t zsa = { dp, zh };
        zilog_t *zilog;

        ASSERT(spa_writeable(dp->dp_spa));

        /*
         * We only want to visit blocks that have been claimed
         * but not yet replayed.
         */
        if (claim_txg == 0)
                return;

        zilog = zil_alloc(dp->dp_meta_objset, zh);

        (void) zil_parse(zilog, dsl_scan_zil_block, dsl_scan_zil_record, &zsa,
            claim_txg);

        zil_free(zilog);
}

/* ARGSUSED */
static void
dsl_scan_prefetch(dsl_scan_t *scn, arc_buf_t *buf, blkptr_t *bp,
    uint64_t objset, uint64_t object, uint64_t blkid)
{
        zbookmark_phys_t czb;
        arc_flags_t flags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;

        if (zfs_no_scrub_prefetch)
                return;

        if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_min_txg ||
            (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE))
                return;

        SET_BOOKMARK(&czb, objset, object, BP_GET_LEVEL(bp), blkid);

        (void) arc_read(scn->scn_zio_root, scn->scn_dp->dp_spa, bp,
            NULL, NULL, ZIO_PRIORITY_ASYNC_READ,
            ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD, &flags, &czb);
}

static boolean_t
dsl_scan_check_resume(dsl_scan_t *scn, const dnode_phys_t *dnp,
    const zbookmark_phys_t *zb)
{
        /*
         * We never skip over user/group accounting objects (obj<0)
         */
        if (!ZB_IS_ZERO(&scn->scn_phys.scn_bookmark) &&
            (int64_t)zb->zb_object >= 0) {
                /*
                 * If we already visited this bp & everything below (in
                 * a prior txg sync), don't bother doing it again.
                 */
                if (zbookmark_subtree_completed(dnp, zb,
                    &scn->scn_phys.scn_bookmark))
                        return (B_TRUE);

                /*
                 * If we found the block we're trying to resume from, or
                 * we went past it to a different object, zero it out to
                 * indicate that it's OK to start checking for suspending
                 * again.
                 */
                if (bcmp(zb, &scn->scn_phys.scn_bookmark, sizeof (*zb)) == 0 ||
                    zb->zb_object > scn->scn_phys.scn_bookmark.zb_object) {
                        dprintf("resuming at %llx/%llx/%llx/%llx\n",
                            (longlong_t)zb->zb_objset,
                            (longlong_t)zb->zb_object,
                            (longlong_t)zb->zb_level,
                            (longlong_t)zb->zb_blkid);
                        bzero(&scn->scn_phys.scn_bookmark, sizeof (*zb));
                }
        }
        return (B_FALSE);
}

/*
 * Return nonzero on i/o error.
 * Return new buf to write out in *bufp.
 */
static int
dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
    dnode_phys_t *dnp, const blkptr_t *bp,
    const zbookmark_phys_t *zb, dmu_tx_t *tx)
{
        dsl_pool_t *dp = scn->scn_dp;
        int zio_flags = ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD;
        int err;

        if (BP_GET_LEVEL(bp) > 0) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                int i;
                blkptr_t *cbp;
                int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
                arc_buf_t *buf;

                err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
                }
                for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
                        dsl_scan_prefetch(scn, buf, cbp, zb->zb_objset,
                            zb->zb_object, zb->zb_blkid * epb + i);
                }
                for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
                        zbookmark_phys_t czb;

                        SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
                            zb->zb_level - 1,
                            zb->zb_blkid * epb + i);
                        dsl_scan_visitbp(cbp, &czb, dnp,
                            ds, scn, ostype, tx);
                }
                arc_buf_destroy(buf, &buf);
        } else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                dnode_phys_t *cdnp;
                int i, j;
                int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
                arc_buf_t *buf;

                err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
                }
                for (i = 0, cdnp = buf->b_data; i < epb; i++, cdnp++) {
                        for (j = 0; j < cdnp->dn_nblkptr; j++) {
                                blkptr_t *cbp = &cdnp->dn_blkptr[j];
                                dsl_scan_prefetch(scn, buf, cbp,
                                    zb->zb_objset, zb->zb_blkid * epb + i, j);
                        }
                }
                for (i = 0, cdnp = buf->b_data; i < epb; i++, cdnp++) {
                        dsl_scan_visitdnode(scn, ds, ostype,
                            cdnp, zb->zb_blkid * epb + i, tx);
                }

                arc_buf_destroy(buf, &buf);
        } else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                objset_phys_t *osp;
                arc_buf_t *buf;

                err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
                }

                osp = buf->b_data;

                dsl_scan_visitdnode(scn, ds, osp->os_type,
                    &osp->os_meta_dnode, DMU_META_DNODE_OBJECT, tx);

                if (OBJSET_BUF_HAS_USERUSED(buf)) {
                        /*
                         * We also always visit user/group accounting
                         * objects, and never skip them, even if we are
                         * suspending.  This is necessary so that the space
                         * deltas from this txg get integrated.
                         */
                        dsl_scan_visitdnode(scn, ds, osp->os_type,
                            &osp->os_groupused_dnode,
                            DMU_GROUPUSED_OBJECT, tx);
                        dsl_scan_visitdnode(scn, ds, osp->os_type,
                            &osp->os_userused_dnode,
                            DMU_USERUSED_OBJECT, tx);
                }
                arc_buf_destroy(buf, &buf);
        }

        return (0);
}

static void
dsl_scan_visitdnode(dsl_scan_t *scn, dsl_dataset_t *ds,
    dmu_objset_type_t ostype, dnode_phys_t *dnp,
    uint64_t object, dmu_tx_t *tx)
{
        int j;

        for (j = 0; j < dnp->dn_nblkptr; j++) {
                zbookmark_phys_t czb;

                SET_BOOKMARK(&czb, ds ? ds->ds_object : 0, object,
                    dnp->dn_nlevels - 1, j);
                dsl_scan_visitbp(&dnp->dn_blkptr[j],
                    &czb, dnp, ds, scn, ostype, tx);
        }

        if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
                zbookmark_phys_t czb;
                SET_BOOKMARK(&czb, ds ? ds->ds_object : 0, object,
                    0, DMU_SPILL_BLKID);
                dsl_scan_visitbp(&dnp->dn_spill,
                    &czb, dnp, ds, scn, ostype, tx);
        }
}

/*
 * The arguments are in this order because mdb can only print the
 * first 5; we want them to be useful.
 */
static void
dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
    dnode_phys_t *dnp, dsl_dataset_t *ds, dsl_scan_t *scn,
    dmu_objset_type_t ostype, dmu_tx_t *tx)
{
        dsl_pool_t *dp = scn->scn_dp;
        arc_buf_t *buf = NULL;
        blkptr_t bp_toread = *bp;

        /* ASSERT(pbuf == NULL || arc_released(pbuf)); */

        if (dsl_scan_check_suspend(scn, zb))
                return;

        if (dsl_scan_check_resume(scn, dnp, zb))
                return;

        if (BP_IS_HOLE(bp))
                return;

        scn->scn_visited_this_txg++;

        dprintf_bp(bp,
            "visiting ds=%p/%llu zb=%llx/%llx/%llx/%llx bp=%p",
            ds, ds ? ds->ds_object : 0,
            zb->zb_objset, zb->zb_object, zb->zb_level, zb->zb_blkid,
            bp);

        if (bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
                return;

        if (dsl_scan_recurse(scn, ds, ostype, dnp, &bp_toread, zb, tx) != 0)
                return;

        /*
         * If dsl_scan_ddt() has already visited this block, it will have
         * already done any translations or scrubbing, so don't call the
         * callback again.
         */
        if (ddt_class_contains(dp->dp_spa,
            scn->scn_phys.scn_ddt_class_max, bp)) {
                ASSERT(buf == NULL);
                return;
        }

        /*
         * If this block is from the future (after cur_max_txg), then we
         * are doing this on behalf of a deleted snapshot, and we will
         * revisit the future block on the next pass of this dataset.
         * Don't scan it now unless we need to because something
         * under it was modified.
         */
        if (BP_PHYSICAL_BIRTH(bp) <= scn->scn_phys.scn_cur_max_txg) {
                scan_funcs[scn->scn_phys.scn_func](dp, bp, zb);
        }
}

static void
dsl_scan_visit_rootbp(dsl_scan_t *scn, dsl_dataset_t *ds, blkptr_t *bp,
    dmu_tx_t *tx)
{
        zbookmark_phys_t zb;

        SET_BOOKMARK(&zb, ds ? ds->ds_object : DMU_META_OBJSET,
            ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
        dsl_scan_visitbp(bp, &zb, NULL,
            ds, scn, DMU_OST_NONE, tx);

        dprintf_ds(ds, "finished scan%s", "");
}

void
dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
{
        dsl_pool_t *dp = ds->ds_dir->dd_pool;
        dsl_scan_t *scn = dp->dp_scan;
        uint64_t mintxg;

        if (scn->scn_phys.scn_state != DSS_SCANNING)
                return;

        if (scn->scn_phys.scn_bookmark.zb_objset == ds->ds_object) {
                if (ds->ds_is_snapshot) {
                        /*
                         * Note:
                         *  - scn_cur_{min,max}_txg stays the same.
                         *  - Setting the flag is not really necessary if
                         *    scn_cur_max_txg == scn_max_txg, because there
                         *    is nothing after this snapshot that we care
                         *    about.  However, we set it anyway and then
                         *    ignore it when we retraverse it in
                         *    dsl_scan_visitds().
                         */
                        scn->scn_phys.scn_bookmark.zb_objset =
                            dsl_dataset_phys(ds)->ds_next_snap_obj;
                        zfs_dbgmsg("destroying ds %llu; currently traversing; "
                            "reset zb_objset to %llu",
                            (u_longlong_t)ds->ds_object,
                            (u_longlong_t)dsl_dataset_phys(ds)->
                            ds_next_snap_obj);
                        scn->scn_phys.scn_flags |= DSF_VISIT_DS_AGAIN;
                } else {
                        SET_BOOKMARK(&scn->scn_phys.scn_bookmark,
                            ZB_DESTROYED_OBJSET, 0, 0, 0);
                        zfs_dbgmsg("destroying ds %llu; currently traversing; "
                            "reset bookmark to -1,0,0,0",
                            (u_longlong_t)ds->ds_object);
                }
        } else if (zap_lookup_int_key(dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj, ds->ds_object, &mintxg) == 0) {
                ASSERT3U(dsl_dataset_phys(ds)->ds_num_children, <=, 1);
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
                if (ds->ds_is_snapshot) {
                        /*
                         * We keep the same mintxg; it could be >
                         * ds_creation_txg if the previous snapshot was
                         * deleted too.
                         */
                        VERIFY(zap_add_int_key(dp->dp_meta_objset,
                            scn->scn_phys.scn_queue_obj,
                            dsl_dataset_phys(ds)->ds_next_snap_obj,
                            mintxg, tx) == 0);
                        zfs_dbgmsg("destroying ds %llu; in queue; "
                            "replacing with %llu",
                            (u_longlong_t)ds->ds_object,
                            (u_longlong_t)dsl_dataset_phys(ds)->
                            ds_next_snap_obj);
                } else {
                        zfs_dbgmsg("destroying ds %llu; in queue; removing",
                            (u_longlong_t)ds->ds_object);
                }
        }

        /*
         * dsl_scan_sync() should be called after this, and should sync
         * out our changed state, but just to be safe, do it here.
         */
        dsl_scan_sync_state(scn, tx);
}

void
dsl_scan_ds_snapshotted(dsl_dataset_t *ds, dmu_tx_t *tx)
{
        dsl_pool_t *dp = ds->ds_dir->dd_pool;
        dsl_scan_t *scn = dp->dp_scan;
        uint64_t mintxg;

        if (scn->scn_phys.scn_state != DSS_SCANNING)
                return;

        ASSERT(dsl_dataset_phys(ds)->ds_prev_snap_obj != 0);

        if (scn->scn_phys.scn_bookmark.zb_objset == ds->ds_object) {
                scn->scn_phys.scn_bookmark.zb_objset =
                    dsl_dataset_phys(ds)->ds_prev_snap_obj;
                zfs_dbgmsg("snapshotting ds %llu; currently traversing; "
                    "reset zb_objset to %llu",
                    (u_longlong_t)ds->ds_object,
                    (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
        } else if (zap_lookup_int_key(dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj, ds->ds_object, &mintxg) == 0) {
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
                VERIFY(zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, mintxg, tx) == 0);
                zfs_dbgmsg("snapshotting ds %llu; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds->ds_object,
                    (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
        }
        dsl_scan_sync_state(scn, tx);
}

void
dsl_scan_ds_clone_swapped(dsl_dataset_t *ds1, dsl_dataset_t *ds2, dmu_tx_t *tx)
{
        dsl_pool_t *dp = ds1->ds_dir->dd_pool;
        dsl_scan_t *scn = dp->dp_scan;
        uint64_t mintxg;

        if (scn->scn_phys.scn_state != DSS_SCANNING)
                return;

        if (scn->scn_phys.scn_bookmark.zb_objset == ds1->ds_object) {
                scn->scn_phys.scn_bookmark.zb_objset = ds2->ds_object;
                zfs_dbgmsg("clone_swap ds %llu; currently traversing; "
                    "reset zb_objset to %llu",
                    (u_longlong_t)ds1->ds_object,
                    (u_longlong_t)ds2->ds_object);
        } else if (scn->scn_phys.scn_bookmark.zb_objset == ds2->ds_object) {
                scn->scn_phys.scn_bookmark.zb_objset = ds1->ds_object;
                zfs_dbgmsg("clone_swap ds %llu; currently traversing; "
                    "reset zb_objset to %llu",
                    (u_longlong_t)ds2->ds_object,
                    (u_longlong_t)ds1->ds_object);
        }

        if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
            ds1->ds_object, &mintxg) == 0) {
                int err;

                ASSERT3U(mintxg, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
                ASSERT3U(mintxg, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds1->ds_object, tx));
                err = zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds2->ds_object, mintxg, tx);
                VERIFY(err == 0 || err == EEXIST);
                if (err == EEXIST) {
                        /* Both were there to begin with */
                        VERIFY(0 == zap_add_int_key(dp->dp_meta_objset,
                            scn->scn_phys.scn_queue_obj,
                            ds1->ds_object, mintxg, tx));
                }
                zfs_dbgmsg("clone_swap ds %llu; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds1->ds_object,
                    (u_longlong_t)ds2->ds_object);
        } else if (zap_lookup_int_key(dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj, ds2->ds_object, &mintxg) == 0) {
                ASSERT3U(mintxg, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
                ASSERT3U(mintxg, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds2->ds_object, tx));
                VERIFY(0 == zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds1->ds_object, mintxg, tx));
                zfs_dbgmsg("clone_swap ds %llu; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds2->ds_object,
                    (u_longlong_t)ds1->ds_object);
        }

        dsl_scan_sync_state(scn, tx);
}

struct enqueue_clones_arg {
        dmu_tx_t *tx;
        uint64_t originobj;
};

/* ARGSUSED */
static int
enqueue_clones_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
{
        struct enqueue_clones_arg *eca = arg;
        dsl_dataset_t *ds;
        int err;
        dsl_scan_t *scn = dp->dp_scan;

        if (dsl_dir_phys(hds->ds_dir)->dd_origin_obj != eca->originobj)
                return (0);

        err = dsl_dataset_hold_obj(dp, hds->ds_object, FTAG, &ds);
        if (err)
                return (err);

        while (dsl_dataset_phys(ds)->ds_prev_snap_obj != eca->originobj) {
                dsl_dataset_t *prev;
                err = dsl_dataset_hold_obj(dp,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, FTAG, &prev);

                dsl_dataset_rele(ds, FTAG);
                if (err)
                        return (err);
                ds = prev;
        }
        VERIFY(zap_add_int_key(dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj, ds->ds_object,
            dsl_dataset_phys(ds)->ds_prev_snap_txg, eca->tx) == 0);
        dsl_dataset_rele(ds, FTAG);
        return (0);
}

static void
dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
{
        dsl_pool_t *dp = scn->scn_dp;
        dsl_dataset_t *ds;

        VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));

        if (scn->scn_phys.scn_cur_min_txg >=
            scn->scn_phys.scn_max_txg) {
                /*
                 * This can happen if this snapshot was created after the
                 * scan started, and we already completed a previous snapshot
                 * that was created after the scan started.  This snapshot
                 * only references blocks with:
                 *
                 *      birth < our ds_creation_txg
                 *      cur_min_txg is no less than ds_creation_txg.
                 *      We have already visited these blocks.
                 * or
                 *      birth > scn_max_txg
                 *      The scan requested not to visit these blocks.
                 *
                 * Subsequent snapshots (and clones) can reference our
                 * blocks, or blocks with even higher birth times.
                 * Therefore we do not need to visit them either,
                 * so we do not add them to the work queue.
                 *
                 * Note that checking for cur_min_txg >= cur_max_txg
                 * is not sufficient, because in that case we may need to
                 * visit subsequent snapshots.  This happens when min_txg > 0,
                 * which raises cur_min_txg.  In this case we will visit
                 * this dataset but skip all of its blocks, because the
                 * rootbp's birth time is < cur_min_txg.  Then we will
                 * add the next snapshots/clones to the work queue.
                 */
                char *dsname = kmem_alloc(MAXNAMELEN, KM_SLEEP);
                dsl_dataset_name(ds, dsname);
                zfs_dbgmsg("scanning dataset %llu (%s) is unnecessary because "
                    "cur_min_txg (%llu) >= max_txg (%llu)",
                    dsobj, dsname,
                    scn->scn_phys.scn_cur_min_txg,
                    scn->scn_phys.scn_max_txg);
                kmem_free(dsname, MAXNAMELEN);

                goto out;
        }

        /*
         * Only the ZIL in the head (non-snapshot) is valid. Even though
         * snapshots can have ZIL block pointers (which may be the same
         * BP as in the head), they must be ignored. In addition, $ORIGIN
         * doesn't have a objset (i.e. its ds_bp is a hole) so we don't
         * need to look for a ZIL in it either. So we traverse the ZIL here,
         * rather than in scan_recurse(), because the regular snapshot
         * block-sharing rules don't apply to it.
         */
        if (DSL_SCAN_IS_SCRUB_RESILVER(scn) && !dsl_dataset_is_snapshot(ds) &&
            ds->ds_dir != dp->dp_origin_snap->ds_dir) {
                objset_t *os;
                if (dmu_objset_from_ds(ds, &os) != 0) {
                        goto out;
                }
                dsl_scan_zil(dp, &os->os_zil_header);
        }

        /*
         * Iterate over the bps in this ds.
         */
        dmu_buf_will_dirty(ds->ds_dbuf, tx);
        rrw_enter(&ds->ds_bp_rwlock, RW_READER, FTAG);
        dsl_scan_visit_rootbp(scn, ds, &dsl_dataset_phys(ds)->ds_bp, tx);
        rrw_exit(&ds->ds_bp_rwlock, FTAG);

        char *dsname = kmem_alloc(ZFS_MAX_DATASET_NAME_LEN, KM_SLEEP);
        dsl_dataset_name(ds, dsname);
        zfs_dbgmsg("scanned dataset %llu (%s) with min=%llu max=%llu; "
            "suspending=%u",
            (longlong_t)dsobj, dsname,
            (longlong_t)scn->scn_phys.scn_cur_min_txg,
            (longlong_t)scn->scn_phys.scn_cur_max_txg,
            (int)scn->scn_suspending);
        kmem_free(dsname, ZFS_MAX_DATASET_NAME_LEN);

        if (scn->scn_suspending)
                goto out;

        /*
         * We've finished this pass over this dataset.
         */

        /*
         * If we did not completely visit this dataset, do another pass.
         */
        if (scn->scn_phys.scn_flags & DSF_VISIT_DS_AGAIN) {
                zfs_dbgmsg("incomplete pass; visiting again");
                scn->scn_phys.scn_flags &= ~DSF_VISIT_DS_AGAIN;
                VERIFY(zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds->ds_object,
                    scn->scn_phys.scn_cur_max_txg, tx) == 0);
                goto out;
        }

        /*
         * Add descendent datasets to work queue.
         */
        if (dsl_dataset_phys(ds)->ds_next_snap_obj != 0) {
                VERIFY(zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj,
                    dsl_dataset_phys(ds)->ds_next_snap_obj,
                    dsl_dataset_phys(ds)->ds_creation_txg, tx) == 0);
        }
        if (dsl_dataset_phys(ds)->ds_num_children > 1) {
                boolean_t usenext = B_FALSE;
                if (dsl_dataset_phys(ds)->ds_next_clones_obj != 0) {
                        uint64_t count;
                        /*
                         * A bug in a previous version of the code could
                         * cause upgrade_clones_cb() to not set
                         * ds_next_snap_obj when it should, leading to a
                         * missing entry.  Therefore we can only use the
                         * next_clones_obj when its count is correct.
                         */
                        int err = zap_count(dp->dp_meta_objset,
                            dsl_dataset_phys(ds)->ds_next_clones_obj, &count);
                        if (err == 0 &&
                            count == dsl_dataset_phys(ds)->ds_num_children - 1)
                                usenext = B_TRUE;
                }

                if (usenext) {
                        VERIFY0(zap_join_key(dp->dp_meta_objset,
                            dsl_dataset_phys(ds)->ds_next_clones_obj,
                            scn->scn_phys.scn_queue_obj,
                            dsl_dataset_phys(ds)->ds_creation_txg, tx));
                } else {
                        struct enqueue_clones_arg eca;
                        eca.tx = tx;
                        eca.originobj = ds->ds_object;

                        VERIFY0(dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
                            enqueue_clones_cb, &eca, DS_FIND_CHILDREN));
                }
        }

out:
        dsl_dataset_rele(ds, FTAG);
}

/* ARGSUSED */
static int
enqueue_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
{
        dmu_tx_t *tx = arg;
        dsl_dataset_t *ds;
        int err;
        dsl_scan_t *scn = dp->dp_scan;

        err = dsl_dataset_hold_obj(dp, hds->ds_object, FTAG, &ds);
        if (err)
                return (err);

        while (dsl_dataset_phys(ds)->ds_prev_snap_obj != 0) {
                dsl_dataset_t *prev;
                err = dsl_dataset_hold_obj(dp,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, FTAG, &prev);
                if (err) {
                        dsl_dataset_rele(ds, FTAG);
                        return (err);
                }

                /*
                 * If this is a clone, we don't need to worry about it for now.
                 */
                if (dsl_dataset_phys(prev)->ds_next_snap_obj != ds->ds_object) {
                        dsl_dataset_rele(ds, FTAG);
                        dsl_dataset_rele(prev, FTAG);
                        return (0);
                }
                dsl_dataset_rele(ds, FTAG);
                ds = prev;
        }

        VERIFY(zap_add_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
            ds->ds_object, dsl_dataset_phys(ds)->ds_prev_snap_txg, tx) == 0);
        dsl_dataset_rele(ds, FTAG);
        return (0);
}

/*
 * Scrub/dedup interaction.
 *
 * If there are N references to a deduped block, we don't want to scrub it
 * N times -- ideally, we should scrub it exactly once.
 *
 * We leverage the fact that the dde's replication class (enum ddt_class)
 * is ordered from highest replication class (DDT_CLASS_DITTO) to lowest
 * (DDT_CLASS_UNIQUE) so that we may walk the DDT in that order.
 *
 * To prevent excess scrubbing, the scrub begins by walking the DDT
 * to find all blocks with refcnt > 1, and scrubs each of these once.
 * Since there are two replication classes which contain blocks with
 * refcnt > 1, we scrub the highest replication class (DDT_CLASS_DITTO) first.
 * Finally the top-down scrub begins, only visiting blocks with refcnt == 1.
 *
 * There would be nothing more to say if a block's refcnt couldn't change
 * during a scrub, but of course it can so we must account for changes
 * in a block's replication class.
 *
 * Here's an example of what can occur:
 *
 * If a block has refcnt > 1 during the DDT scrub phase, but has refcnt == 1
 * when visited during the top-down scrub phase, it will be scrubbed twice.
 * This negates our scrub optimization, but is otherwise harmless.
 *
 * If a block has refcnt == 1 during the DDT scrub phase, but has refcnt > 1
 * on each visit during the top-down scrub phase, it will never be scrubbed.
 * To catch this, ddt_sync_entry() notifies the scrub code whenever a block's
 * reference class transitions to a higher level (i.e DDT_CLASS_UNIQUE to
 * DDT_CLASS_DUPLICATE); if it transitions from refcnt == 1 to refcnt > 1
 * while a scrub is in progress, it scrubs the block right then.
 */
static void
dsl_scan_ddt(dsl_scan_t *scn, dmu_tx_t *tx)
{
        ddt_bookmark_t *ddb = &scn->scn_phys.scn_ddt_bookmark;
        ddt_entry_t dde = { 0 };
        int error;
        uint64_t n = 0;

        while ((error = ddt_walk(scn->scn_dp->dp_spa, ddb, &dde)) == 0) {
                ddt_t *ddt;

                if (ddb->ddb_class > scn->scn_phys.scn_ddt_class_max)
                        break;
                dprintf("visiting ddb=%llu/%llu/%llu/%llx\n",
                    (longlong_t)ddb->ddb_class,
                    (longlong_t)ddb->ddb_type,
                    (longlong_t)ddb->ddb_checksum,
                    (longlong_t)ddb->ddb_cursor);

                /* There should be no pending changes to the dedup table */
                ddt = scn->scn_dp->dp_spa->spa_ddt[ddb->ddb_checksum];
                ASSERT(avl_first(&ddt->ddt_tree) == NULL);

                dsl_scan_ddt_entry(scn, ddb->ddb_checksum, &dde, tx);
                n++;

                if (dsl_scan_check_suspend(scn, NULL))
                        break;
        }

        zfs_dbgmsg("scanned %llu ddt entries with class_max = %u; "
            "suspending=%u", (longlong_t)n,
            (int)scn->scn_phys.scn_ddt_class_max, (int)scn->scn_suspending);

        ASSERT(error == 0 || error == ENOENT);
        ASSERT(error != ENOENT ||
            ddb->ddb_class > scn->scn_phys.scn_ddt_class_max);
}

/* ARGSUSED */
void
dsl_scan_ddt_entry(dsl_scan_t *scn, enum zio_checksum checksum,
    ddt_entry_t *dde, dmu_tx_t *tx)
{
        const ddt_key_t *ddk = &dde->dde_key;
        ddt_phys_t *ddp = dde->dde_phys;
        blkptr_t bp;
        zbookmark_phys_t zb = { 0 };

        if (scn->scn_phys.scn_state != DSS_SCANNING)
                return;

        for (int p = 0; p < DDT_PHYS_TYPES; p++, ddp++) {
                if (ddp->ddp_phys_birth == 0 ||
                    ddp->ddp_phys_birth > scn->scn_phys.scn_max_txg)
                        continue;
                ddt_bp_create(checksum, ddk, ddp, &bp);

                scn->scn_visited_this_txg++;
                scan_funcs[scn->scn_phys.scn_func](scn->scn_dp, &bp, &zb);
        }
}

static void
dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
{
        dsl_pool_t *dp = scn->scn_dp;
        zap_cursor_t zc;
        zap_attribute_t za;

        if (scn->scn_phys.scn_ddt_bookmark.ddb_class <=
            scn->scn_phys.scn_ddt_class_max) {
                scn->scn_phys.scn_cur_min_txg = scn->scn_phys.scn_min_txg;
                scn->scn_phys.scn_cur_max_txg = scn->scn_phys.scn_max_txg;
                dsl_scan_ddt(scn, tx);
                if (scn->scn_suspending)
                        return;
        }

        if (scn->scn_phys.scn_bookmark.zb_objset == DMU_META_OBJSET) {
                /* First do the MOS & ORIGIN */

                scn->scn_phys.scn_cur_min_txg = scn->scn_phys.scn_min_txg;
                scn->scn_phys.scn_cur_max_txg = scn->scn_phys.scn_max_txg;
                dsl_scan_visit_rootbp(scn, NULL,
                    &dp->dp_meta_rootbp, tx);
                spa_set_rootblkptr(dp->dp_spa, &dp->dp_meta_rootbp);
                if (scn->scn_suspending)
                        return;

                if (spa_version(dp->dp_spa) < SPA_VERSION_DSL_SCRUB) {
                        VERIFY0(dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
                            enqueue_cb, tx, DS_FIND_CHILDREN));
                } else {
                        dsl_scan_visitds(scn,
                            dp->dp_origin_snap->ds_object, tx);
                }
                ASSERT(!scn->scn_suspending);
        } else if (scn->scn_phys.scn_bookmark.zb_objset !=
            ZB_DESTROYED_OBJSET) {
                /*
                 * If we were suspended, continue from here.  Note if the
                 * ds we were suspended on was deleted, the zb_objset may
                 * be -1, so we will skip this and find a new objset
                 * below.
                 */
                dsl_scan_visitds(scn, scn->scn_phys.scn_bookmark.zb_objset, tx);
                if (scn->scn_suspending)
                        return;
        }

        /*
         * In case we were suspended right at the end of the ds, zero the
         * bookmark so we don't think that we're still trying to resume.
         */
        bzero(&scn->scn_phys.scn_bookmark, sizeof (zbookmark_phys_t));

        /* keep pulling things out of the zap-object-as-queue */
        while (zap_cursor_init(&zc, dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj),
            zap_cursor_retrieve(&zc, &za) == 0) {
                dsl_dataset_t *ds;
                uint64_t dsobj;

                dsobj = zfs_strtonum(za.za_name, NULL);
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, dsobj, tx));

                /* Set up min/max txg */
                VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
                if (za.za_first_integer != 0) {
                        scn->scn_phys.scn_cur_min_txg =
                            MAX(scn->scn_phys.scn_min_txg,
                            za.za_first_integer);
                } else {
                        scn->scn_phys.scn_cur_min_txg =
                            MAX(scn->scn_phys.scn_min_txg,
                            dsl_dataset_phys(ds)->ds_prev_snap_txg);
                }
                scn->scn_phys.scn_cur_max_txg = dsl_scan_ds_maxtxg(ds);
                dsl_dataset_rele(ds, FTAG);

                dsl_scan_visitds(scn, dsobj, tx);
                zap_cursor_fini(&zc);
                if (scn->scn_suspending)
                        return;
        }
        zap_cursor_fini(&zc);
}

static boolean_t
dsl_scan_async_block_should_pause(dsl_scan_t *scn)
{
        uint64_t elapsed_nanosecs;

        if (zfs_recover)
                return (B_FALSE);

        if (scn->scn_visited_this_txg >= zfs_async_block_max_blocks)
                return (B_TRUE);

        elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
        return (elapsed_nanosecs / NANOSEC > zfs_txg_timeout ||
            (NSEC2MSEC(elapsed_nanosecs) > scn->scn_async_block_min_time_ms &&
            txg_sync_waiting(scn->scn_dp)) ||
            spa_shutting_down(scn->scn_dp->dp_spa));
}

static int
dsl_scan_free_block_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
{
        dsl_scan_t *scn = arg;

        if (!scn->scn_is_bptree ||
            (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_OBJSET)) {
                if (dsl_scan_async_block_should_pause(scn))
                        return (SET_ERROR(ERESTART));
        }

        zio_nowait(zio_free_sync(scn->scn_zio_root, scn->scn_dp->dp_spa,
            dmu_tx_get_txg(tx), bp, 0));
        dsl_dir_diduse_space(tx->tx_pool->dp_free_dir, DD_USED_HEAD,
            -bp_get_dsize_sync(scn->scn_dp->dp_spa, bp),
            -BP_GET_PSIZE(bp), -BP_GET_UCSIZE(bp), tx);
        scn->scn_visited_this_txg++;
        return (0);
}

static int
dsl_scan_obsolete_block_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
{
        dsl_scan_t *scn = arg;
        const dva_t *dva = &bp->blk_dva[0];

        if (dsl_scan_async_block_should_pause(scn))
                return (SET_ERROR(ERESTART));

        spa_vdev_indirect_mark_obsolete(scn->scn_dp->dp_spa,
            DVA_GET_VDEV(dva), DVA_GET_OFFSET(dva),
            DVA_GET_ASIZE(dva), tx);
        scn->scn_visited_this_txg++;
        return (0);
}

boolean_t
dsl_scan_active(dsl_scan_t *scn)
{
        spa_t *spa = scn->scn_dp->dp_spa;
        uint64_t used = 0, comp, uncomp;

        if (spa->spa_load_state != SPA_LOAD_NONE)
                return (B_FALSE);
        if (spa_shutting_down(spa))
                return (B_FALSE);
        if ((scn->scn_phys.scn_state == DSS_SCANNING &&
            !dsl_scan_is_paused_scrub(scn)) ||
            (scn->scn_async_destroying && !scn->scn_async_stalled))
                return (B_TRUE);

        if (spa_version(scn->scn_dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
                (void) bpobj_space(&scn->scn_dp->dp_free_bpobj,
                    &used, &comp, &uncomp);
        }
        return (used != 0);
}

static int
dsl_process_async_destroys(dsl_pool_t *dp, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dp->dp_scan;
        spa_t *spa = dp->dp_spa;
        int err = 0;

        if (spa_suspend_async_destroy(spa))
                return (0);

        if (zfs_free_bpobj_enabled &&
            spa_version(dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
                scn->scn_is_bptree = B_FALSE;
                scn->scn_async_block_min_time_ms = zfs_free_min_time_ms;
                scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
                    NULL, ZIO_FLAG_MUSTSUCCEED);
                err = bpobj_iterate(&dp->dp_free_bpobj,
                    dsl_scan_free_block_cb, scn, tx);
                VERIFY3U(0, ==, zio_wait(scn->scn_zio_root));

                if (err != 0 && err != ERESTART)
                        zfs_panic_recover("error %u from bpobj_iterate()", err);
        }

        if (err == 0 && spa_feature_is_active(spa, SPA_FEATURE_ASYNC_DESTROY)) {
                ASSERT(scn->scn_async_destroying);
                scn->scn_is_bptree = B_TRUE;
                scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
                    NULL, ZIO_FLAG_MUSTSUCCEED);
                err = bptree_iterate(dp->dp_meta_objset,
                    dp->dp_bptree_obj, B_TRUE, dsl_scan_free_block_cb, scn, tx);
                VERIFY0(zio_wait(scn->scn_zio_root));

                if (err == EIO || err == ECKSUM) {
                        err = 0;
                } else if (err != 0 && err != ERESTART) {
                        zfs_panic_recover("error %u from "
                            "traverse_dataset_destroyed()", err);
                }

                if (bptree_is_empty(dp->dp_meta_objset, dp->dp_bptree_obj)) {
                        /* finished; deactivate async destroy feature */
                        spa_feature_decr(spa, SPA_FEATURE_ASYNC_DESTROY, tx);
                        ASSERT(!spa_feature_is_active(spa,
                            SPA_FEATURE_ASYNC_DESTROY));
                        VERIFY0(zap_remove(dp->dp_meta_objset,
                            DMU_POOL_DIRECTORY_OBJECT,
                            DMU_POOL_BPTREE_OBJ, tx));
                        VERIFY0(bptree_free(dp->dp_meta_objset,
                            dp->dp_bptree_obj, tx));
                        dp->dp_bptree_obj = 0;
                        scn->scn_async_destroying = B_FALSE;
                        scn->scn_async_stalled = B_FALSE;
                } else {
                        /*
                         * If we didn't make progress, mark the async
                         * destroy as stalled, so that we will not initiate
                         * a spa_sync() on its behalf.  Note that we only
                         * check this if we are not finished, because if the
                         * bptree had no blocks for us to visit, we can
                         * finish without "making progress".
                         */
                        scn->scn_async_stalled =
                            (scn->scn_visited_this_txg == 0);
                }
        }
        if (scn->scn_visited_this_txg) {
                zfs_dbgmsg("freed %llu blocks in %llums from "
                    "free_bpobj/bptree txg %llu; err=%u",
                    (longlong_t)scn->scn_visited_this_txg,
                    (longlong_t)
                    NSEC2MSEC(gethrtime() - scn->scn_sync_start_time),
                    (longlong_t)tx->tx_txg, err);
                scn->scn_visited_this_txg = 0;

                /*
                 * Write out changes to the DDT that may be required as a
                 * result of the blocks freed.  This ensures that the DDT
                 * is clean when a scrub/resilver runs.
                 */
                ddt_sync(spa, tx->tx_txg);
        }
        if (err != 0)
                return (err);
        if (dp->dp_free_dir != NULL && !scn->scn_async_destroying &&
            zfs_free_leak_on_eio &&
            (dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes != 0 ||
            dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes != 0 ||
            dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes != 0)) {
                /*
                 * We have finished background destroying, but there is still
                 * some space left in the dp_free_dir. Transfer this leaked
                 * space to the dp_leak_dir.
                 */
                if (dp->dp_leak_dir == NULL) {
                        rrw_enter(&dp->dp_config_rwlock, RW_WRITER, FTAG);
                        (void) dsl_dir_create_sync(dp, dp->dp_root_dir,
                            LEAK_DIR_NAME, tx);
                        VERIFY0(dsl_pool_open_special_dir(dp,
                            LEAK_DIR_NAME, &dp->dp_leak_dir));
                        rrw_exit(&dp->dp_config_rwlock, FTAG);
                }
                dsl_dir_diduse_space(dp->dp_leak_dir, DD_USED_HEAD,
                    dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes,
                    dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes,
                    dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes, tx);
                dsl_dir_diduse_space(dp->dp_free_dir, DD_USED_HEAD,
                    -dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes,
                    -dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes,
                    -dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes, tx);
        }

        if (dp->dp_free_dir != NULL && !scn->scn_async_destroying) {
                /* finished; verify that space accounting went to zero */
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes);
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes);
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes);
        }

        EQUIV(bpobj_is_open(&dp->dp_obsolete_bpobj),
            0 == zap_contains(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_OBSOLETE_BPOBJ));
        if (err == 0 && bpobj_is_open(&dp->dp_obsolete_bpobj)) {
                ASSERT(spa_feature_is_active(dp->dp_spa,
                    SPA_FEATURE_OBSOLETE_COUNTS));

                scn->scn_is_bptree = B_FALSE;
                scn->scn_async_block_min_time_ms = zfs_obsolete_min_time_ms;
                err = bpobj_iterate(&dp->dp_obsolete_bpobj,
                    dsl_scan_obsolete_block_cb, scn, tx);
                if (err != 0 && err != ERESTART)
                        zfs_panic_recover("error %u from bpobj_iterate()", err);

                if (bpobj_is_empty(&dp->dp_obsolete_bpobj))
                        dsl_pool_destroy_obsolete_bpobj(dp, tx);
        }

        return (0);
}

void
dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dp->dp_scan;
        spa_t *spa = dp->dp_spa;
        int err = 0;

        /*
         * Check for scn_restart_txg before checking spa_load_state, so
         * that we can restart an old-style scan while the pool is being
         * imported (see dsl_scan_init).
         */
        if (dsl_scan_restarting(scn, tx)) {
                pool_scan_func_t func = POOL_SCAN_SCRUB;
                dsl_scan_done(scn, B_FALSE, tx);
                if (vdev_resilver_needed(spa->spa_root_vdev, NULL, NULL))
                        func = POOL_SCAN_RESILVER;
                zfs_dbgmsg("restarting scan func=%u txg=%llu",
                    func, tx->tx_txg);
                dsl_scan_setup_sync(&func, tx);
        }

        /*
         * Only process scans in sync pass 1.
         */
        if (spa_sync_pass(dp->dp_spa) > 1)
                return;

        /*
         * If the spa is shutting down, then stop scanning. This will
         * ensure that the scan does not dirty any new data during the
         * shutdown phase.
         */
        if (spa_shutting_down(spa))
                return;

        /*
         * If the scan is inactive due to a stalled async destroy, try again.
         */
        if (!scn->scn_async_stalled && !dsl_scan_active(scn))
                return;

        scn->scn_visited_this_txg = 0;
        scn->scn_suspending = B_FALSE;
        scn->scn_sync_start_time = gethrtime();
        spa->spa_scrub_active = B_TRUE;

        /*
         * First process the async destroys.  If we pause, don't do
         * any scrubbing or resilvering.  This ensures that there are no
         * async destroys while we are scanning, so the scan code doesn't
         * have to worry about traversing it.  It is also faster to free the
         * blocks than to scrub them.
         */
        err = dsl_process_async_destroys(dp, tx);
        if (err != 0)
                return;

        if (scn->scn_phys.scn_state != DSS_SCANNING)
                return;

        if (scn->scn_done_txg == tx->tx_txg) {
                ASSERT(!scn->scn_suspending);
                /* finished with scan. */
                zfs_dbgmsg("txg %llu scan complete", tx->tx_txg);
                dsl_scan_done(scn, B_TRUE, tx);
                ASSERT3U(spa->spa_scrub_inflight, ==, 0);
                dsl_scan_sync_state(scn, tx);
                return;
        }

        if (dsl_scan_is_paused_scrub(scn))
                return;

        if (scn->scn_phys.scn_ddt_bookmark.ddb_class <=
            scn->scn_phys.scn_ddt_class_max) {
                zfs_dbgmsg("doing scan sync txg %llu; "
                    "ddt bm=%llu/%llu/%llu/%llx",
                    (longlong_t)tx->tx_txg,
                    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_class,
                    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_type,
                    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_checksum,
                    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_cursor);
                ASSERT(scn->scn_phys.scn_bookmark.zb_objset == 0);
                ASSERT(scn->scn_phys.scn_bookmark.zb_object == 0);
                ASSERT(scn->scn_phys.scn_bookmark.zb_level == 0);
                ASSERT(scn->scn_phys.scn_bookmark.zb_blkid == 0);
        } else {
                zfs_dbgmsg("doing scan sync txg %llu; bm=%llu/%llu/%llu/%llu",
                    (longlong_t)tx->tx_txg,
                    (longlong_t)scn->scn_phys.scn_bookmark.zb_objset,
                    (longlong_t)scn->scn_phys.scn_bookmark.zb_object,
                    (longlong_t)scn->scn_phys.scn_bookmark.zb_level,
                    (longlong_t)scn->scn_phys.scn_bookmark.zb_blkid);
        }

        scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
            NULL, ZIO_FLAG_CANFAIL);
        dsl_pool_config_enter(dp, FTAG);
        dsl_scan_visit(scn, tx);
        dsl_pool_config_exit(dp, FTAG);
        (void) zio_wait(scn->scn_zio_root);
        scn->scn_zio_root = NULL;

        zfs_dbgmsg("visited %llu blocks in %llums",
            (longlong_t)scn->scn_visited_this_txg,
            (longlong_t)NSEC2MSEC(gethrtime() - scn->scn_sync_start_time));

        if (!scn->scn_suspending) {
                scn->scn_done_txg = tx->tx_txg + 1;
                zfs_dbgmsg("txg %llu traversal complete, waiting till txg %llu",
                    tx->tx_txg, scn->scn_done_txg);
        }

        if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
                mutex_enter(&spa->spa_scrub_lock);
                while (spa->spa_scrub_inflight > 0) {
                        cv_wait(&spa->spa_scrub_io_cv,
                            &spa->spa_scrub_lock);
                }
                mutex_exit(&spa->spa_scrub_lock);
        }

        dsl_scan_sync_state(scn, tx);
}

/*
 * This will start a new scan, or restart an existing one.
 */
void
dsl_resilver_restart(dsl_pool_t *dp, uint64_t txg)
{
        if (txg == 0) {
                dmu_tx_t *tx;
                tx = dmu_tx_create_dd(dp->dp_mos_dir);
                VERIFY(0 == dmu_tx_assign(tx, TXG_WAIT));

                txg = dmu_tx_get_txg(tx);
                dp->dp_scan->scn_restart_txg = txg;
                dmu_tx_commit(tx);
        } else {
                dp->dp_scan->scn_restart_txg = txg;
        }
        zfs_dbgmsg("restarting resilver txg=%llu", txg);
}

boolean_t
dsl_scan_resilvering(dsl_pool_t *dp)
{
        return (dp->dp_scan->scn_phys.scn_state == DSS_SCANNING &&
            dp->dp_scan->scn_phys.scn_func == POOL_SCAN_RESILVER);
}

/*
 * scrub consumers
 */

static void
count_block(zfs_all_blkstats_t *zab, const blkptr_t *bp)
{
        int i;

        /*
         * If we resume after a reboot, zab will be NULL; don't record
         * incomplete stats in that case.
         */
        if (zab == NULL)
                return;

        for (i = 0; i < 4; i++) {
                int l = (i < 2) ? BP_GET_LEVEL(bp) : DN_MAX_LEVELS;
                int t = (i & 1) ? BP_GET_TYPE(bp) : DMU_OT_TOTAL;
                if (t & DMU_OT_NEWTYPE)
                        t = DMU_OT_OTHER;
                zfs_blkstat_t *zb = &zab->zab_type[l][t];
                int equal;

                zb->zb_count++;
                zb->zb_asize += BP_GET_ASIZE(bp);
                zb->zb_lsize += BP_GET_LSIZE(bp);
                zb->zb_psize += BP_GET_PSIZE(bp);
                zb->zb_gangs += BP_COUNT_GANG(bp);

                switch (BP_GET_NDVAS(bp)) {
                case 2:
                        if (DVA_GET_VDEV(&bp->blk_dva[0]) ==
                            DVA_GET_VDEV(&bp->blk_dva[1]))
                                zb->zb_ditto_2_of_2_samevdev++;
                        break;
                case 3:
                        equal = (DVA_GET_VDEV(&bp->blk_dva[0]) ==
                            DVA_GET_VDEV(&bp->blk_dva[1])) +
                            (DVA_GET_VDEV(&bp->blk_dva[0]) ==
                            DVA_GET_VDEV(&bp->blk_dva[2])) +
                            (DVA_GET_VDEV(&bp->blk_dva[1]) ==
                            DVA_GET_VDEV(&bp->blk_dva[2]));
                        if (equal == 1)
                                zb->zb_ditto_2_of_3_samevdev++;
                        else if (equal == 3)
                                zb->zb_ditto_3_of_3_samevdev++;
                        break;
                }
        }
}

static void
dsl_scan_scrub_done(zio_t *zio)
{
        spa_t *spa = zio->io_spa;

        abd_free(zio->io_abd);

        mutex_enter(&spa->spa_scrub_lock);
        spa->spa_scrub_inflight--;
        cv_broadcast(&spa->spa_scrub_io_cv);

        if (zio->io_error && (zio->io_error != ECKSUM ||
            !(zio->io_flags & ZIO_FLAG_SPECULATIVE))) {
                spa->spa_dsl_pool->dp_scan->scn_phys.scn_errors++;
        }
        mutex_exit(&spa->spa_scrub_lock);
}

static int
dsl_scan_scrub_cb(dsl_pool_t *dp,
    const blkptr_t *bp, const zbookmark_phys_t *zb)
{
        dsl_scan_t *scn = dp->dp_scan;
        size_t size = BP_GET_PSIZE(bp);
        spa_t *spa = dp->dp_spa;
        uint64_t phys_birth = BP_PHYSICAL_BIRTH(bp);
        boolean_t needs_io;
        int zio_flags = ZIO_FLAG_SCAN_THREAD | ZIO_FLAG_RAW | ZIO_FLAG_CANFAIL;
        int scan_delay = 0;

        if (phys_birth <= scn->scn_phys.scn_min_txg ||
            phys_birth >= scn->scn_phys.scn_max_txg)
                return (0);

        count_block(dp->dp_blkstats, bp);

        if (BP_IS_EMBEDDED(bp))
                return (0);

        ASSERT(DSL_SCAN_IS_SCRUB_RESILVER(scn));
        if (scn->scn_phys.scn_func == POOL_SCAN_SCRUB) {
                zio_flags |= ZIO_FLAG_SCRUB;
                needs_io = B_TRUE;
                scan_delay = zfs_scrub_delay;
        } else {
                ASSERT3U(scn->scn_phys.scn_func, ==, POOL_SCAN_RESILVER);
                zio_flags |= ZIO_FLAG_RESILVER;
                needs_io = B_FALSE;
                scan_delay = zfs_resilver_delay;
        }

        /* If it's an intent log block, failure is expected. */
        if (zb->zb_level == ZB_ZIL_LEVEL)
                zio_flags |= ZIO_FLAG_SPECULATIVE;

        for (int d = 0; d < BP_GET_NDVAS(bp); d++) {
                vdev_t *vd = vdev_lookup_top(spa,
                    DVA_GET_VDEV(&bp->blk_dva[d]));

                /*
                 * Keep track of how much data we've examined so that
                 * zpool(1M) status can make useful progress reports.
                 */
                scn->scn_phys.scn_examined += DVA_GET_ASIZE(&bp->blk_dva[d]);
                spa->spa_scan_pass_exam += DVA_GET_ASIZE(&bp->blk_dva[d]);

                /* if it's a resilver, this may not be in the target range */
                if (!needs_io) {
                        if (vd->vdev_ops == &vdev_indirect_ops) {
                                /*
                                 * The indirect vdev can point to multiple
                                 * vdevs.  For simplicity, always create
                                 * the resilver zio_t. zio_vdev_io_start()
                                 * will bypass the child resilver i/o's if
                                 * they are on vdevs that don't have DTL's.
                                 */
                                needs_io = B_TRUE;
                        } else if (DVA_GET_GANG(&bp->blk_dva[d])) {
                                /*
                                 * Gang members may be spread across multiple
                                 * vdevs, so the best estimate we have is the
                                 * scrub range, which has already been checked.
                                 * XXX -- it would be better to change our
                                 * allocation policy to ensure that all
                                 * gang members reside on the same vdev.
                                 */
                                needs_io = B_TRUE;
                        } else {
                                needs_io = vdev_dtl_contains(vd, DTL_PARTIAL,
                                    phys_birth, 1);
                        }
                }
        }

        if (needs_io && !zfs_no_scrub_io) {
                vdev_t *rvd = spa->spa_root_vdev;
                uint64_t maxinflight = rvd->vdev_children * zfs_top_maxinflight;

                mutex_enter(&spa->spa_scrub_lock);
                while (spa->spa_scrub_inflight >= maxinflight)
                        cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
                spa->spa_scrub_inflight++;
                mutex_exit(&spa->spa_scrub_lock);

                /*
                 * If we're seeing recent (zfs_scan_idle) "important" I/Os
                 * then throttle our workload to limit the impact of a scan.
                 */
                if (ddi_get_lbolt64() - spa->spa_last_io <= zfs_scan_idle)
                        delay(scan_delay);

                zio_nowait(zio_read(NULL, spa, bp,
                    abd_alloc_for_io(size, B_FALSE), size, dsl_scan_scrub_done,
                    NULL, ZIO_PRIORITY_SCRUB, zio_flags, zb));
        }

        /* do not relocate this block */
        return (0);
}

/*
 * Called by the ZFS_IOC_POOL_SCAN ioctl to start a scrub or resilver.
 * Can also be called to resume a paused scrub.
 */
int
dsl_scan(dsl_pool_t *dp, pool_scan_func_t func)
{
        spa_t *spa = dp->dp_spa;
        dsl_scan_t *scn = dp->dp_scan;

        /*
         * Purge all vdev caches and probe all devices.  We do this here
         * rather than in sync context because this requires a writer lock
         * on the spa_config lock, which we can't do from sync context.  The
         * spa_scrub_reopen flag indicates that vdev_open() should not
         * attempt to start another scrub.
         */
        spa_vdev_state_enter(spa, SCL_NONE);
        spa->spa_scrub_reopen = B_TRUE;
        vdev_reopen(spa->spa_root_vdev);
        spa->spa_scrub_reopen = B_FALSE;
        (void) spa_vdev_state_exit(spa, NULL, 0);

        if (func == POOL_SCAN_SCRUB && dsl_scan_is_paused_scrub(scn)) {
                /* got scrub start cmd, resume paused scrub */
                int err = dsl_scrub_set_pause_resume(scn->scn_dp,
                    POOL_SCRUB_NORMAL);
                if (err == 0) {
                        spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_RESUME);
                        return (ECANCELED);
                }

                return (SET_ERROR(err));
        }

        return (dsl_sync_task(spa_name(spa), dsl_scan_setup_check,
            dsl_scan_setup_sync, &func, 0, ZFS_SPACE_CHECK_EXTRA_RESERVED));
}

static boolean_t
dsl_scan_restarting(dsl_scan_t *scn, dmu_tx_t *tx)
{
        return (scn->scn_restart_txg != 0 &&
            scn->scn_restart_txg <= tx->tx_txg);
}