fs/zfs/dsl_pool.c

	dsl_pool.c revision 0a586cea3ceec7e5e50e7e54c745082a7a333ac2
/*
 * 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 2010 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#include <sys/dsl_pool.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_synctask.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>

int zfs_no_write_throttle = 0;
int zfs_write_limit_shift = 3;          /* 1/8th of physical memory */
int zfs_txg_synctime_ms = 5000;     /* target millisecs to sync a txg */

uint64_t zfs_write_limit_min = 32 << 20;    /* min write limit is 32MB */
uint64_t zfs_write_limit_max = 0;       /* max data payload per txg */
uint64_t zfs_write_limit_inflated = 0;
uint64_t zfs_write_limit_override = 0;

kmutex_t zfs_write_limit_lock;

static pgcnt_t old_physmem = 0;

static int
dsl_pool_open_special_dir(dsl_pool_t *dp, const char *name, dsl_dir_t **ddp)
{
    uint64_t obj;
    int err;

    err = zap_lookup(dp->dp_meta_objset,
        dp->dp_root_dir->dd_phys->dd_child_dir_zapobj,
        name, sizeof (obj), 1, &obj);
    if (err)
        return (err);

    return (dsl_dir_open_obj(dp, obj, name, dp, ddp));
}

static dsl_pool_t *
dsl_pool_open_impl(spa_t *spa, uint64_t txg)
{
    dsl_pool_t *dp;
    blkptr_t *bp = spa_get_rootblkptr(spa);

    dp = kmem_zalloc(sizeof (dsl_pool_t), KM_SLEEP);
    dp->dp_spa = spa;
    dp->dp_meta_rootbp = *bp;
    rw_init(&dp->dp_config_rwlock, NULL, RW_DEFAULT, NULL);
    dp->dp_write_limit = zfs_write_limit_min;
    txg_init(dp, txg);

    txg_list_create(&dp->dp_dirty_datasets,
        offsetof(dsl_dataset_t, ds_dirty_link));
    txg_list_create(&dp->dp_dirty_dirs,
        offsetof(dsl_dir_t, dd_dirty_link));
    txg_list_create(&dp->dp_sync_tasks,
        offsetof(dsl_sync_task_group_t, dstg_node));
    list_create(&dp->dp_synced_datasets, sizeof (dsl_dataset_t),
        offsetof(dsl_dataset_t, ds_synced_link));

    mutex_init(&dp->dp_lock, NULL, MUTEX_DEFAULT, NULL);
    mutex_init(&dp->dp_scrub_cancel_lock, NULL, MUTEX_DEFAULT, NULL);

    dp->dp_vnrele_taskq = taskq_create("zfs_vn_rele_taskq", 1, minclsyspri,
        1, 4, 0);

    return (dp);
}

int
dsl_pool_open(spa_t *spa, uint64_t txg, dsl_pool_t **dpp)
{
    int err;
    dsl_pool_t *dp = dsl_pool_open_impl(spa, txg);
    dsl_dir_t *dd;
    dsl_dataset_t *ds;

    rw_enter(&dp->dp_config_rwlock, RW_WRITER);
    err = dmu_objset_open_impl(spa, NULL, &dp->dp_meta_rootbp,
        &dp->dp_meta_objset);
    if (err)
        goto out;

    err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
        DMU_POOL_ROOT_DATASET, sizeof (uint64_t), 1,
        &dp->dp_root_dir_obj);
    if (err)
        goto out;

    err = dsl_dir_open_obj(dp, dp->dp_root_dir_obj,
        NULL, dp, &dp->dp_root_dir);
    if (err)
        goto out;

    err = dsl_pool_open_special_dir(dp, MOS_DIR_NAME, &dp->dp_mos_dir);
    if (err)
        goto out;

    if (spa_version(spa) >= SPA_VERSION_ORIGIN) {
        err = dsl_pool_open_special_dir(dp, ORIGIN_DIR_NAME, &dd);
        if (err)
            goto out;
        err = dsl_dataset_hold_obj(dp, dd->dd_phys->dd_head_dataset_obj,
            FTAG, &ds);
        if (err == 0) {
            err = dsl_dataset_hold_obj(dp,
                ds->ds_phys->ds_prev_snap_obj, dp,
                &dp->dp_origin_snap);
            dsl_dataset_rele(ds, FTAG);
        }
        dsl_dir_close(dd, dp);
        if (err)
            goto out;
    }

    err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
        DMU_POOL_TMP_USERREFS, sizeof (uint64_t), 1,
        &dp->dp_tmp_userrefs_obj);
    if (err == ENOENT)
        err = 0;
    if (err)
        goto out;

    /* get scrub status */
    err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
        DMU_POOL_SCRUB_FUNC, sizeof (uint32_t), 1,
        &dp->dp_scrub_func);
    if (err == 0) {
        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_SCRUB_QUEUE, sizeof (uint64_t), 1,
            &dp->dp_scrub_queue_obj);
        if (err)
            goto out;
        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_SCRUB_MIN_TXG, sizeof (uint64_t), 1,
            &dp->dp_scrub_min_txg);
        if (err)
            goto out;
        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_SCRUB_MAX_TXG, sizeof (uint64_t), 1,
            &dp->dp_scrub_max_txg);
        if (err)
            goto out;
        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_SCRUB_BOOKMARK, sizeof (uint64_t),
            sizeof (dp->dp_scrub_bookmark) / sizeof (uint64_t),
            &dp->dp_scrub_bookmark);
        if (err)
            goto out;
        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_SCRUB_DDT_BOOKMARK, sizeof (uint64_t),
            sizeof (dp->dp_scrub_ddt_bookmark) / sizeof (uint64_t),
            &dp->dp_scrub_ddt_bookmark);
        if (err && err != ENOENT)
            goto out;
        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_SCRUB_DDT_CLASS_MAX, sizeof (uint64_t), 1,
            &dp->dp_scrub_ddt_class_max);
        if (err && err != ENOENT)
            goto out;
        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_SCRUB_ERRORS, sizeof (uint64_t), 1,
            &spa->spa_scrub_errors);
        if (err)
            goto out;
        if (spa_version(spa) < SPA_VERSION_DSL_SCRUB) {
            /*
             * A new-type scrub was in progress on an old
             * pool.  Restart from the beginning, since the
             * old software may have changed the pool in the
             * meantime.
             */
            dsl_pool_scrub_restart(dp);
        }
    } else {
        /*
         * It's OK if there is no scrub in progress (and if
         * there was an I/O error, ignore it).
         */
        err = 0;
    }

out:
    rw_exit(&dp->dp_config_rwlock);
    if (err)
        dsl_pool_close(dp);
    else
        *dpp = dp;

    return (err);
}

void
dsl_pool_close(dsl_pool_t *dp)
{
    /* drop our references from dsl_pool_open() */

    /*
     * Since we held the origin_snap from "syncing" context (which
     * includes pool-opening context), it actually only got a "ref"
     * and not a hold, so just drop that here.
     */
    if (dp->dp_origin_snap)
        dsl_dataset_drop_ref(dp->dp_origin_snap, dp);
    if (dp->dp_mos_dir)
        dsl_dir_close(dp->dp_mos_dir, dp);
    if (dp->dp_root_dir)
        dsl_dir_close(dp->dp_root_dir, dp);

    /* undo the dmu_objset_open_impl(mos) from dsl_pool_open() */
    if (dp->dp_meta_objset)
        dmu_objset_evict(dp->dp_meta_objset);

    txg_list_destroy(&dp->dp_dirty_datasets);
    txg_list_destroy(&dp->dp_sync_tasks);
    txg_list_destroy(&dp->dp_dirty_dirs);
    list_destroy(&dp->dp_synced_datasets);

    arc_flush(dp->dp_spa);
    txg_fini(dp);
    rw_destroy(&dp->dp_config_rwlock);
    mutex_destroy(&dp->dp_lock);
    mutex_destroy(&dp->dp_scrub_cancel_lock);
    taskq_destroy(dp->dp_vnrele_taskq);
    if (dp->dp_blkstats)
        kmem_free(dp->dp_blkstats, sizeof (zfs_all_blkstats_t));
    kmem_free(dp, sizeof (dsl_pool_t));
}

dsl_pool_t *
dsl_pool_create(spa_t *spa, nvlist_t *zplprops, uint64_t txg)
{
    int err;
    dsl_pool_t *dp = dsl_pool_open_impl(spa, txg);
    dmu_tx_t *tx = dmu_tx_create_assigned(dp, txg);
    objset_t *os;
    dsl_dataset_t *ds;
    uint64_t dsobj;

    /* create and open the MOS (meta-objset) */
    dp->dp_meta_objset = dmu_objset_create_impl(spa,
        NULL, &dp->dp_meta_rootbp, DMU_OST_META, tx);

    /* create the pool directory */
    err = zap_create_claim(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
        DMU_OT_OBJECT_DIRECTORY, DMU_OT_NONE, 0, tx);
    ASSERT3U(err, ==, 0);

    /* create and open the root dir */
    dp->dp_root_dir_obj = dsl_dir_create_sync(dp, NULL, NULL, tx);
    VERIFY(0 == dsl_dir_open_obj(dp, dp->dp_root_dir_obj,
        NULL, dp, &dp->dp_root_dir));

    /* create and open the meta-objset dir */
    (void) dsl_dir_create_sync(dp, dp->dp_root_dir, MOS_DIR_NAME, tx);
    VERIFY(0 == dsl_pool_open_special_dir(dp,
        MOS_DIR_NAME, &dp->dp_mos_dir));

    if (spa_version(spa) >= SPA_VERSION_DSL_SCRUB)
        dsl_pool_create_origin(dp, tx);

    /* create the root dataset */
    dsobj = dsl_dataset_create_sync_dd(dp->dp_root_dir, NULL, 0, tx);

    /* create the root objset */
    VERIFY(0 == dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
    os = dmu_objset_create_impl(dp->dp_spa, ds,
        dsl_dataset_get_blkptr(ds), DMU_OST_ZFS, tx);
#ifdef _KERNEL
    zfs_create_fs(os, kcred, zplprops, tx);
#endif
    dsl_dataset_rele(ds, FTAG);

    dmu_tx_commit(tx);

    return (dp);
}

void
dsl_pool_sync(dsl_pool_t *dp, uint64_t txg)
{
    zio_t *zio;
    dmu_tx_t *tx;
    dsl_dir_t *dd;
    dsl_dataset_t *ds;
    dsl_sync_task_group_t *dstg;
    objset_t *mos = dp->dp_meta_objset;
    hrtime_t start, write_time;
    uint64_t data_written;
    int err;

    tx = dmu_tx_create_assigned(dp, txg);

    dp->dp_read_overhead = 0;
    start = gethrtime();

    zio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
    while (ds = txg_list_remove(&dp->dp_dirty_datasets, txg)) {
        /*
         * We must not sync any non-MOS datasets twice, because
         * we may have taken a snapshot of them.  However, we
         * may sync newly-created datasets on pass 2.
         */
        ASSERT(!list_link_active(&ds->ds_synced_link));
        list_insert_tail(&dp->dp_synced_datasets, ds);
        dsl_dataset_sync(ds, zio, tx);
    }
    DTRACE_PROBE(pool_sync__1setup);
    err = zio_wait(zio);

    write_time = gethrtime() - start;
    ASSERT(err == 0);
    DTRACE_PROBE(pool_sync__2rootzio);

    for (ds = list_head(&dp->dp_synced_datasets); ds;
        ds = list_next(&dp->dp_synced_datasets, ds))
        dmu_objset_do_userquota_updates(ds->ds_objset, tx);

    /*
     * Sync the datasets again to push out the changes due to
     * userquota updates.  This must be done before we process the
     * sync tasks, because that could cause a snapshot of a dataset
     * whose ds_bp will be rewritten when we do this 2nd sync.
     */
    zio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
    while (ds = txg_list_remove(&dp->dp_dirty_datasets, txg)) {
        ASSERT(list_link_active(&ds->ds_synced_link));
        dmu_buf_rele(ds->ds_dbuf, ds);
        dsl_dataset_sync(ds, zio, tx);
    }
    err = zio_wait(zio);

    /*
     * If anything was added to a deadlist during a zio done callback,
     * it had to be put on the deferred queue.  Enqueue it for real now.
     */
    for (ds = list_head(&dp->dp_synced_datasets); ds;
        ds = list_next(&dp->dp_synced_datasets, ds))
        bplist_sync(&ds->ds_deadlist,
            bplist_enqueue_cb, &ds->ds_deadlist, tx);

    while (dstg = txg_list_remove(&dp->dp_sync_tasks, txg)) {
        /*
         * No more sync tasks should have been added while we
         * were syncing.
         */
        ASSERT(spa_sync_pass(dp->dp_spa) == 1);
        dsl_sync_task_group_sync(dstg, tx);
    }
    DTRACE_PROBE(pool_sync__3task);

    start = gethrtime();
    while (dd = txg_list_remove(&dp->dp_dirty_dirs, txg))
        dsl_dir_sync(dd, tx);
    write_time += gethrtime() - start;

    if (spa_sync_pass(dp->dp_spa) == 1) {
        dp->dp_scrub_prefetch_zio_root = zio_root(dp->dp_spa, NULL,
            NULL, ZIO_FLAG_CANFAIL);
        dsl_pool_scrub_sync(dp, tx);
        (void) zio_wait(dp->dp_scrub_prefetch_zio_root);
    }

    start = gethrtime();
    if (list_head(&mos->os_dirty_dnodes[txg & TXG_MASK]) != NULL ||
        list_head(&mos->os_free_dnodes[txg & TXG_MASK]) != NULL) {
        zio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
        dmu_objset_sync(mos, zio, tx);
        err = zio_wait(zio);
        ASSERT(err == 0);
        dprintf_bp(&dp->dp_meta_rootbp, "meta objset rootbp is %s", "");
        spa_set_rootblkptr(dp->dp_spa, &dp->dp_meta_rootbp);
    }
    write_time += gethrtime() - start;
    DTRACE_PROBE2(pool_sync__4io, hrtime_t, write_time,
        hrtime_t, dp->dp_read_overhead);
    write_time -= dp->dp_read_overhead;

    dmu_tx_commit(tx);

    data_written = dp->dp_space_towrite[txg & TXG_MASK];
    dp->dp_space_towrite[txg & TXG_MASK] = 0;
    ASSERT(dp->dp_tempreserved[txg & TXG_MASK] == 0);

    /*
     * If the write limit max has not been explicitly set, set it
     * to a fraction of available physical memory (default 1/8th).
     * Note that we must inflate the limit because the spa
     * inflates write sizes to account for data replication.
     * Check this each sync phase to catch changing memory size.
     */
    if (physmem != old_physmem && zfs_write_limit_shift) {
        mutex_enter(&zfs_write_limit_lock);
        old_physmem = physmem;
        zfs_write_limit_max = ptob(physmem) >> zfs_write_limit_shift;
        zfs_write_limit_inflated = MAX(zfs_write_limit_min,
            spa_get_asize(dp->dp_spa, zfs_write_limit_max));
        mutex_exit(&zfs_write_limit_lock);
    }

    /*
     * Attempt to keep the sync time consistent by adjusting the
     * amount of write traffic allowed into each transaction group.
     * Weight the throughput calculation towards the current value:
     *  thru = 3/4 old_thru + 1/4 new_thru
     *
     * Note: write_time is in nanosecs, so write_time/MICROSEC
     * yields millisecs
     */
    ASSERT(zfs_write_limit_min > 0);
    if (data_written > zfs_write_limit_min / 8 && write_time > MICROSEC) {
        uint64_t throughput = data_written / (write_time / MICROSEC);

        if (dp->dp_throughput)
            dp->dp_throughput = throughput / 4 +
                3 * dp->dp_throughput / 4;
        else
            dp->dp_throughput = throughput;
        dp->dp_write_limit = MIN(zfs_write_limit_inflated,
            MAX(zfs_write_limit_min,
            dp->dp_throughput * zfs_txg_synctime_ms));
    }
}

void
dsl_pool_sync_done(dsl_pool_t *dp, uint64_t txg)
{
    dsl_dataset_t *ds;
    objset_t *os;

    while (ds = list_head(&dp->dp_synced_datasets)) {
        list_remove(&dp->dp_synced_datasets, ds);
        os = ds->ds_objset;
        zil_clean(os->os_zil);
        ASSERT(!dmu_objset_is_dirty(os, txg));
        dmu_buf_rele(ds->ds_dbuf, ds);
    }
    ASSERT(!dmu_objset_is_dirty(dp->dp_meta_objset, txg));
}

/*
 * TRUE if the current thread is the tx_sync_thread or if we
 * are being called from SPA context during pool initialization.
 */
int
dsl_pool_sync_context(dsl_pool_t *dp)
{
    return (curthread == dp->dp_tx.tx_sync_thread ||
        spa_get_dsl(dp->dp_spa) == NULL);
}

uint64_t
dsl_pool_adjustedsize(dsl_pool_t *dp, boolean_t netfree)
{
    uint64_t space, resv;

    /*
     * Reserve about 1.6% (1/64), or at least 32MB, for allocation
     * efficiency.
     * XXX The intent log is not accounted for, so it must fit
     * within this slop.
     *
     * If we're trying to assess whether it's OK to do a free,
     * cut the reservation in half to allow forward progress
     * (e.g. make it possible to rm(1) files from a full pool).
     */
    space = spa_get_dspace(dp->dp_spa);
    resv = MAX(space >> 6, SPA_MINDEVSIZE >> 1);
    if (netfree)
        resv >>= 1;

    return (space - resv);
}

int
dsl_pool_tempreserve_space(dsl_pool_t *dp, uint64_t space, dmu_tx_t *tx)
{
    uint64_t reserved = 0;
    uint64_t write_limit = (zfs_write_limit_override ?
        zfs_write_limit_override : dp->dp_write_limit);

    if (zfs_no_write_throttle) {
        atomic_add_64(&dp->dp_tempreserved[tx->tx_txg & TXG_MASK],
            space);
        return (0);
    }

    /*
     * Check to see if we have exceeded the maximum allowed IO for
     * this transaction group.  We can do this without locks since
     * a little slop here is ok.  Note that we do the reserved check
     * with only half the requested reserve: this is because the
     * reserve requests are worst-case, and we really don't want to
     * throttle based off of worst-case estimates.
     */
    if (write_limit > 0) {
        reserved = dp->dp_space_towrite[tx->tx_txg & TXG_MASK]
            + dp->dp_tempreserved[tx->tx_txg & TXG_MASK] / 2;

        if (reserved && reserved > write_limit)
            return (ERESTART);
    }

    atomic_add_64(&dp->dp_tempreserved[tx->tx_txg & TXG_MASK], space);

    /*
     * If this transaction group is over 7/8ths capacity, delay
     * the caller 1 clock tick.  This will slow down the "fill"
     * rate until the sync process can catch up with us.
     */
    if (reserved && reserved > (write_limit - (write_limit >> 3)))
        txg_delay(dp, tx->tx_txg, 1);

    return (0);
}

void
dsl_pool_tempreserve_clear(dsl_pool_t *dp, int64_t space, dmu_tx_t *tx)
{
    ASSERT(dp->dp_tempreserved[tx->tx_txg & TXG_MASK] >= space);
    atomic_add_64(&dp->dp_tempreserved[tx->tx_txg & TXG_MASK], -space);
}

void
dsl_pool_memory_pressure(dsl_pool_t *dp)
{
    uint64_t space_inuse = 0;
    int i;

    if (dp->dp_write_limit == zfs_write_limit_min)
        return;

    for (i = 0; i < TXG_SIZE; i++) {
        space_inuse += dp->dp_space_towrite[i];
        space_inuse += dp->dp_tempreserved[i];
    }
    dp->dp_write_limit = MAX(zfs_write_limit_min,
        MIN(dp->dp_write_limit, space_inuse / 4));
}

void
dsl_pool_willuse_space(dsl_pool_t *dp, int64_t space, dmu_tx_t *tx)
{
    if (space > 0) {
        mutex_enter(&dp->dp_lock);
        dp->dp_space_towrite[tx->tx_txg & TXG_MASK] += space;
        mutex_exit(&dp->dp_lock);
    }
}

/* ARGSUSED */
static int
upgrade_clones_cb(spa_t *spa, uint64_t dsobj, const char *dsname, void *arg)
{
    dmu_tx_t *tx = arg;
    dsl_dataset_t *ds, *prev = NULL;
    int err;
    dsl_pool_t *dp = spa_get_dsl(spa);

    err = dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds);
    if (err)
        return (err);

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

        if (prev->ds_phys->ds_next_snap_obj != ds->ds_object)
            break;
        dsl_dataset_rele(ds, FTAG);
        ds = prev;
        prev = NULL;
    }

    if (prev == NULL) {
        prev = dp->dp_origin_snap;

        /*
         * The $ORIGIN can't have any data, or the accounting
         * will be wrong.
         */
        ASSERT(prev->ds_phys->ds_bp.blk_birth == 0);

        /* The origin doesn't get attached to itself */
        if (ds->ds_object == prev->ds_object) {
            dsl_dataset_rele(ds, FTAG);
            return (0);
        }

        dmu_buf_will_dirty(ds->ds_dbuf, tx);
        ds->ds_phys->ds_prev_snap_obj = prev->ds_object;
        ds->ds_phys->ds_prev_snap_txg = prev->ds_phys->ds_creation_txg;

        dmu_buf_will_dirty(ds->ds_dir->dd_dbuf, tx);
        ds->ds_dir->dd_phys->dd_origin_obj = prev->ds_object;

        dmu_buf_will_dirty(prev->ds_dbuf, tx);
        prev->ds_phys->ds_num_children++;

        if (ds->ds_phys->ds_next_snap_obj == 0) {
            ASSERT(ds->ds_prev == NULL);
            VERIFY(0 == dsl_dataset_hold_obj(dp,
                ds->ds_phys->ds_prev_snap_obj, ds, &ds->ds_prev));
        }
    }

    ASSERT(ds->ds_dir->dd_phys->dd_origin_obj == prev->ds_object);
    ASSERT(ds->ds_phys->ds_prev_snap_obj == prev->ds_object);

    if (prev->ds_phys->ds_next_clones_obj == 0) {
        dmu_buf_will_dirty(prev->ds_dbuf, tx);
        prev->ds_phys->ds_next_clones_obj =
            zap_create(dp->dp_meta_objset,
            DMU_OT_NEXT_CLONES, DMU_OT_NONE, 0, tx);
    }
    VERIFY(0 == zap_add_int(dp->dp_meta_objset,
        prev->ds_phys->ds_next_clones_obj, ds->ds_object, tx));

    dsl_dataset_rele(ds, FTAG);
    if (prev != dp->dp_origin_snap)
        dsl_dataset_rele(prev, FTAG);
    return (0);
}

void
dsl_pool_upgrade_clones(dsl_pool_t *dp, dmu_tx_t *tx)
{
    ASSERT(dmu_tx_is_syncing(tx));
    ASSERT(dp->dp_origin_snap != NULL);

    VERIFY3U(0, ==, dmu_objset_find_spa(dp->dp_spa, NULL, upgrade_clones_cb,
        tx, DS_FIND_CHILDREN));
}

void
dsl_pool_create_origin(dsl_pool_t *dp, dmu_tx_t *tx)
{
    uint64_t dsobj;
    dsl_dataset_t *ds;

    ASSERT(dmu_tx_is_syncing(tx));
    ASSERT(dp->dp_origin_snap == NULL);

    /* create the origin dir, ds, & snap-ds */
    rw_enter(&dp->dp_config_rwlock, RW_WRITER);
    dsobj = dsl_dataset_create_sync(dp->dp_root_dir, ORIGIN_DIR_NAME,
        NULL, 0, kcred, tx);
    VERIFY(0 == dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
    dsl_dataset_snapshot_sync(ds, ORIGIN_DIR_NAME, kcred, tx);
    VERIFY(0 == dsl_dataset_hold_obj(dp, ds->ds_phys->ds_prev_snap_obj,
        dp, &dp->dp_origin_snap));
    dsl_dataset_rele(ds, FTAG);
    rw_exit(&dp->dp_config_rwlock);
}

taskq_t *
dsl_pool_vnrele_taskq(dsl_pool_t *dp)
{
    return (dp->dp_vnrele_taskq);
}

/*
 * Walk through the pool-wide zap object of temporary snapshot user holds
 * and release them.
 */
void
dsl_pool_clean_tmp_userrefs(dsl_pool_t *dp)
{
    zap_attribute_t za;
    zap_cursor_t zc;
    objset_t *mos = dp->dp_meta_objset;
    uint64_t zapobj = dp->dp_tmp_userrefs_obj;

    if (zapobj == 0)
        return;
    ASSERT(spa_version(dp->dp_spa) >= SPA_VERSION_USERREFS);

    for (zap_cursor_init(&zc, mos, zapobj);
        zap_cursor_retrieve(&zc, &za) == 0;
        zap_cursor_advance(&zc)) {
        char *htag;
        uint64_t dsobj;

        htag = strchr(za.za_name, '-');
        *htag = '\0';
        ++htag;
        dsobj = strtonum(za.za_name, NULL);
        (void) dsl_dataset_user_release_tmp(dp, dsobj, htag);
    }
    zap_cursor_fini(&zc);
}

/*
 * Create the pool-wide zap object for storing temporary snapshot holds.
 */
void
dsl_pool_user_hold_create_obj(dsl_pool_t *dp, dmu_tx_t *tx)
{
    objset_t *mos = dp->dp_meta_objset;

    ASSERT(dp->dp_tmp_userrefs_obj == 0);
    ASSERT(dmu_tx_is_syncing(tx));

    dp->dp_tmp_userrefs_obj = zap_create(mos, DMU_OT_USERREFS,
        DMU_OT_NONE, 0, tx);

    VERIFY(zap_add(mos, DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_TMP_USERREFS,
        sizeof (uint64_t), 1, &dp->dp_tmp_userrefs_obj, tx) == 0);
}

static int
dsl_pool_user_hold_rele_impl(dsl_pool_t *dp, uint64_t dsobj,
    const char *tag, uint64_t *now, dmu_tx_t *tx, boolean_t holding)
{
    objset_t *mos = dp->dp_meta_objset;
    uint64_t zapobj = dp->dp_tmp_userrefs_obj;
    char *name;
    int error;

    ASSERT(spa_version(dp->dp_spa) >= SPA_VERSION_USERREFS);
    ASSERT(dmu_tx_is_syncing(tx));

    /*
     * If the pool was created prior to SPA_VERSION_USERREFS, the
     * zap object for temporary holds might not exist yet.
     */
    if (zapobj == 0) {
        if (holding) {
            dsl_pool_user_hold_create_obj(dp, tx);
            zapobj = dp->dp_tmp_userrefs_obj;
        } else {
            return (ENOENT);
        }
    }

    name = kmem_asprintf("%llx-%s", (u_longlong_t)dsobj, tag);
    if (holding)
        error = zap_add(mos, zapobj, name, 8, 1, now, tx);
    else
        error = zap_remove(mos, zapobj, name, tx);
    strfree(name);

    return (error);
}

/*
 * Add a temporary hold for the given dataset object and tag.
 */
int
dsl_pool_user_hold(dsl_pool_t *dp, uint64_t dsobj, const char *tag,
    uint64_t *now, dmu_tx_t *tx)
{
    return (dsl_pool_user_hold_rele_impl(dp, dsobj, tag, now, tx, B_TRUE));
}

/*
 * Release a temporary hold for the given dataset object and tag.
 */
int
dsl_pool_user_release(dsl_pool_t *dp, uint64_t dsobj, const char *tag,
    dmu_tx_t *tx)
{
    return (dsl_pool_user_hold_rele_impl(dp, dsobj, tag, NULL,
        tx, B_FALSE));
}