fs/zfs/dmu_traverse.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2012, 2016 by Delphix. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/dmu_objset.h>
#include <sys/dmu_traverse.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_pool.h>
#include <sys/dnode.h>
#include <sys/spa.h>
#include <sys/zio.h>
#include <sys/dmu_impl.h>
#include <sys/sa.h>
#include <sys/sa_impl.h>
#include <sys/callb.h>
#include <sys/zfeature.h>

int32_t zfs_pd_bytes_max = 50 * 1024 * 1024;    /* 50MB */
boolean_t send_holes_without_birth_time = B_TRUE;

typedef struct prefetch_data {
    kmutex_t pd_mtx;
    kcondvar_t pd_cv;
    int32_t pd_bytes_fetched;
    int pd_flags;
    boolean_t pd_cancel;
    boolean_t pd_exited;
    zbookmark_phys_t pd_resume;
} prefetch_data_t;

typedef struct traverse_data {
    spa_t *td_spa;
    uint64_t td_objset;
    blkptr_t *td_rootbp;
    uint64_t td_min_txg;
    zbookmark_phys_t *td_resume;
    int td_flags;
    prefetch_data_t *td_pfd;
    boolean_t td_paused;
    uint64_t td_hole_birth_enabled_txg;
    blkptr_cb_t *td_func;
    void *td_arg;
    boolean_t td_realloc_possible;
} traverse_data_t;

static int traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
    uint64_t objset, uint64_t object);
static void prefetch_dnode_metadata(traverse_data_t *td, const dnode_phys_t *,
    uint64_t objset, uint64_t object);

static int
traverse_zil_block(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
{
    traverse_data_t *td = arg;
    zbookmark_phys_t zb;

    if (BP_IS_HOLE(bp))
        return (0);

    if (claim_txg == 0 && bp->blk_birth >= spa_first_txg(td->td_spa))
        return (0);

    SET_BOOKMARK(&zb, td->td_objset, ZB_ZIL_OBJECT, ZB_ZIL_LEVEL,
        bp->blk_cksum.zc_word[ZIL_ZC_SEQ]);

    (void) td->td_func(td->td_spa, zilog, bp, &zb, NULL, td->td_arg);

    return (0);
}

static int
traverse_zil_record(zilog_t *zilog, lr_t *lrc, void *arg, uint64_t claim_txg)
{
    traverse_data_t *td = arg;

    if (lrc->lrc_txtype == TX_WRITE) {
        lr_write_t *lr = (lr_write_t *)lrc;
        blkptr_t *bp = &lr->lr_blkptr;
        zbookmark_phys_t zb;

        if (BP_IS_HOLE(bp))
            return (0);

        if (claim_txg == 0 || bp->blk_birth < claim_txg)
            return (0);

        SET_BOOKMARK(&zb, td->td_objset, lr->lr_foid,
            ZB_ZIL_LEVEL, lr->lr_offset / BP_GET_LSIZE(bp));

        (void) td->td_func(td->td_spa, zilog, bp, &zb, NULL,
            td->td_arg);
    }
    return (0);
}

static void
traverse_zil(traverse_data_t *td, zil_header_t *zh)
{
    uint64_t claim_txg = zh->zh_claim_txg;
    zilog_t *zilog;

    /*
     * We only want to visit blocks that have been claimed but not yet
     * replayed; plus, in read-only mode, blocks that are already stable.
     */
    if (claim_txg == 0 && spa_writeable(td->td_spa))
        return;

    zilog = zil_alloc(spa_get_dsl(td->td_spa)->dp_meta_objset, zh);

    (void) zil_parse(zilog, traverse_zil_block, traverse_zil_record, td,
        claim_txg);

    zil_free(zilog);
}

typedef enum resume_skip {
    RESUME_SKIP_ALL,
    RESUME_SKIP_NONE,
    RESUME_SKIP_CHILDREN
} resume_skip_t;

/*
 * Returns RESUME_SKIP_ALL if td indicates that we are resuming a traversal and
 * the block indicated by zb does not need to be visited at all. Returns
 * RESUME_SKIP_CHILDREN if we are resuming a post traversal and we reach the
 * resume point. This indicates that this block should be visited but not its
 * children (since they must have been visited in a previous traversal).
 * Otherwise returns RESUME_SKIP_NONE.
 */
static resume_skip_t
resume_skip_check(traverse_data_t *td, const dnode_phys_t *dnp,
    const zbookmark_phys_t *zb)
{
    if (td->td_resume != NULL && !ZB_IS_ZERO(td->td_resume)) {
        /*
         * If we already visited this bp & everything below,
         * don't bother doing it again.
         */
        if (zbookmark_subtree_completed(dnp, zb, td->td_resume))
            return (RESUME_SKIP_ALL);

        /*
         * If we found the block we're trying to resume from, zero
         * the bookmark out to indicate that we have resumed.
         */
        if (bcmp(zb, td->td_resume, sizeof (*zb)) == 0) {
            bzero(td->td_resume, sizeof (*zb));
            if (td->td_flags & TRAVERSE_POST)
                return (RESUME_SKIP_CHILDREN);
        }
    }
    return (RESUME_SKIP_NONE);
}

static void
traverse_prefetch_metadata(traverse_data_t *td,
    const blkptr_t *bp, const zbookmark_phys_t *zb)
{
    arc_flags_t flags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;

    if (!(td->td_flags & TRAVERSE_PREFETCH_METADATA))
        return;
    /*
     * If we are in the process of resuming, don't prefetch, because
     * some children will not be needed (and in fact may have already
     * been freed).
     */
    if (td->td_resume != NULL && !ZB_IS_ZERO(td->td_resume))
        return;
    if (BP_IS_HOLE(bp) || bp->blk_birth <= td->td_min_txg)
        return;
    if (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE)
        return;

    (void) arc_read(NULL, td->td_spa, bp, NULL, NULL,
        ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
}

static boolean_t
prefetch_needed(prefetch_data_t *pfd, const blkptr_t *bp)
{
    ASSERT(pfd->pd_flags & TRAVERSE_PREFETCH_DATA);
    if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp) ||
        BP_GET_TYPE(bp) == DMU_OT_INTENT_LOG)
        return (B_FALSE);
    return (B_TRUE);
}

static int
traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
    const blkptr_t *bp, const zbookmark_phys_t *zb)
{
    zbookmark_phys_t czb;
    int err = 0;
    arc_buf_t *buf = NULL;
    prefetch_data_t *pd = td->td_pfd;
    boolean_t hard = td->td_flags & TRAVERSE_HARD;

    switch (resume_skip_check(td, dnp, zb)) {
    case RESUME_SKIP_ALL:
        return (0);
    case RESUME_SKIP_CHILDREN:
        goto post;
    case RESUME_SKIP_NONE:
        break;
    default:
        ASSERT(0);
    }

    if (bp->blk_birth == 0) {
        /*
         * Since this block has a birth time of 0 it must be one of
         * two things: a hole created before the
         * SPA_FEATURE_HOLE_BIRTH feature was enabled, or a hole
         * which has always been a hole in an object.
         *
         * If a file is written sparsely, then the unwritten parts of
         * the file were "always holes" -- that is, they have been
         * holes since this object was allocated.  However, we (and
         * our callers) can not necessarily tell when an object was
         * allocated.  Therefore, if it's possible that this object
         * was freed and then its object number reused, we need to
         * visit all the holes with birth==0.
         *
         * If it isn't possible that the object number was reused,
         * then if SPA_FEATURE_HOLE_BIRTH was enabled before we wrote
         * all the blocks we will visit as part of this traversal,
         * then this hole must have always existed, so we can skip
         * it.  We visit blocks born after (exclusive) td_min_txg.
         *
         * Note that the meta-dnode cannot be reallocated.
         */
        if (!send_holes_without_birth_time &&
            (!td->td_realloc_possible ||
            zb->zb_object == DMU_META_DNODE_OBJECT) &&
            td->td_hole_birth_enabled_txg <= td->td_min_txg)
            return (0);
    } else if (bp->blk_birth <= td->td_min_txg) {
        return (0);
    }

    if (pd != NULL && !pd->pd_exited && prefetch_needed(pd, bp)) {
        uint64_t size = BP_GET_LSIZE(bp);
        mutex_enter(&pd->pd_mtx);
        ASSERT(pd->pd_bytes_fetched >= 0);
        while (pd->pd_bytes_fetched < size && !pd->pd_exited)
            cv_wait(&pd->pd_cv, &pd->pd_mtx);
        pd->pd_bytes_fetched -= size;
        cv_broadcast(&pd->pd_cv);
        mutex_exit(&pd->pd_mtx);
    }

    if (BP_IS_HOLE(bp)) {
        err = td->td_func(td->td_spa, NULL, bp, zb, dnp, td->td_arg);
        if (err != 0)
            goto post;
        return (0);
    }

    if (td->td_flags & TRAVERSE_PRE) {
        err = td->td_func(td->td_spa, NULL, bp, zb, dnp,
            td->td_arg);
        if (err == TRAVERSE_VISIT_NO_CHILDREN)
            return (0);
        if (err != 0)
            goto post;
    }

    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;

        err = arc_read(NULL, td->td_spa, bp, arc_getbuf_func, &buf,
            ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
        if (err != 0)
            goto post;
        cbp = buf->b_data;

        for (i = 0; i < epb; i++) {
            SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
                zb->zb_level - 1,
                zb->zb_blkid * epb + i);
            traverse_prefetch_metadata(td, &cbp[i], &czb);
        }

        /* recursively visitbp() blocks below this */
        for (i = 0; i < epb; i++) {
            SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
                zb->zb_level - 1,
                zb->zb_blkid * epb + i);
            err = traverse_visitbp(td, dnp, &cbp[i], &czb);
            if (err != 0)
                break;
        }
    } else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
        arc_flags_t flags = ARC_FLAG_WAIT;
        int i;
        int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;

        err = arc_read(NULL, td->td_spa, bp, arc_getbuf_func, &buf,
            ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
        if (err != 0)
            goto post;
        dnode_phys_t *child_dnp = buf->b_data;

        for (i = 0; i < epb; i++) {
            prefetch_dnode_metadata(td, &child_dnp[i],
                zb->zb_objset, zb->zb_blkid * epb + i);
        }

        /* recursively visitbp() blocks below this */
        for (i = 0; i < epb; i++) {
            err = traverse_dnode(td, &child_dnp[i],
                zb->zb_objset, zb->zb_blkid * epb + i);
            if (err != 0)
                break;
        }
    } else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
        arc_flags_t flags = ARC_FLAG_WAIT;

        err = arc_read(NULL, td->td_spa, bp, arc_getbuf_func, &buf,
            ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
        if (err != 0)
            goto post;

        objset_phys_t *osp = buf->b_data;
        prefetch_dnode_metadata(td, &osp->os_meta_dnode, zb->zb_objset,
            DMU_META_DNODE_OBJECT);
        /*
         * See the block comment above for the goal of this variable.
         * If the maxblkid of the meta-dnode is 0, then we know that
         * we've never had more than DNODES_PER_BLOCK objects in the
         * dataset, which means we can't have reused any object ids.
         */
        if (osp->os_meta_dnode.dn_maxblkid == 0)
            td->td_realloc_possible = B_FALSE;

        if (arc_buf_size(buf) >= sizeof (objset_phys_t)) {
            prefetch_dnode_metadata(td, &osp->os_groupused_dnode,
                zb->zb_objset, DMU_GROUPUSED_OBJECT);
            prefetch_dnode_metadata(td, &osp->os_userused_dnode,
                zb->zb_objset, DMU_USERUSED_OBJECT);
        }

        err = traverse_dnode(td, &osp->os_meta_dnode, zb->zb_objset,
            DMU_META_DNODE_OBJECT);
        if (err == 0 && arc_buf_size(buf) >= sizeof (objset_phys_t)) {
            err = traverse_dnode(td, &osp->os_groupused_dnode,
                zb->zb_objset, DMU_GROUPUSED_OBJECT);
        }
        if (err == 0 && arc_buf_size(buf) >= sizeof (objset_phys_t)) {
            err = traverse_dnode(td, &osp->os_userused_dnode,
                zb->zb_objset, DMU_USERUSED_OBJECT);
        }
    }

    if (buf)
        arc_buf_destroy(buf, &buf);

post:
    if (err == 0 && (td->td_flags & TRAVERSE_POST))
        err = td->td_func(td->td_spa, NULL, bp, zb, dnp, td->td_arg);

    if (hard && (err == EIO || err == ECKSUM)) {
        /*
         * Ignore this disk error as requested by the HARD flag,
         * and continue traversal.
         */
        err = 0;
    }

    /*
     * If we are stopping here, set td_resume.
     */
    if (td->td_resume != NULL && err != 0 && !td->td_paused) {
        td->td_resume->zb_objset = zb->zb_objset;
        td->td_resume->zb_object = zb->zb_object;
        td->td_resume->zb_level = 0;
        /*
         * If we have stopped on an indirect block (e.g. due to
         * i/o error), we have not visited anything below it.
         * Set the bookmark to the first level-0 block that we need
         * to visit.  This way, the resuming code does not need to
         * deal with resuming from indirect blocks.
         *
         * Note, if zb_level <= 0, dnp may be NULL, so we don't want
         * to dereference it.
         */
        td->td_resume->zb_blkid = zb->zb_blkid;
        if (zb->zb_level > 0) {
            td->td_resume->zb_blkid <<= zb->zb_level *
                (dnp->dn_indblkshift - SPA_BLKPTRSHIFT);
        }
        td->td_paused = B_TRUE;
    }

    return (err);
}

static void
prefetch_dnode_metadata(traverse_data_t *td, const dnode_phys_t *dnp,
    uint64_t objset, uint64_t object)
{
    int j;
    zbookmark_phys_t czb;

    for (j = 0; j < dnp->dn_nblkptr; j++) {
        SET_BOOKMARK(&czb, objset, object, dnp->dn_nlevels - 1, j);
        traverse_prefetch_metadata(td, &dnp->dn_blkptr[j], &czb);
    }

    if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
        SET_BOOKMARK(&czb, objset, object, 0, DMU_SPILL_BLKID);
        traverse_prefetch_metadata(td, &dnp->dn_spill, &czb);
    }
}

static int
traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
    uint64_t objset, uint64_t object)
{
    int j, err = 0;
    zbookmark_phys_t czb;

    if (object != DMU_META_DNODE_OBJECT && td->td_resume != NULL &&
        object < td->td_resume->zb_object)
        return (0);

    if (td->td_flags & TRAVERSE_PRE) {
        SET_BOOKMARK(&czb, objset, object, ZB_DNODE_LEVEL,
            ZB_DNODE_BLKID);
        err = td->td_func(td->td_spa, NULL, NULL, &czb, dnp,
            td->td_arg);
        if (err == TRAVERSE_VISIT_NO_CHILDREN)
            return (0);
        if (err != 0)
            return (err);
    }

    for (j = 0; j < dnp->dn_nblkptr; j++) {
        SET_BOOKMARK(&czb, objset, object, dnp->dn_nlevels - 1, j);
        err = traverse_visitbp(td, dnp, &dnp->dn_blkptr[j], &czb);
        if (err != 0)
            break;
    }

    if (err == 0 && (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR)) {
        SET_BOOKMARK(&czb, objset, object, 0, DMU_SPILL_BLKID);
        err = traverse_visitbp(td, dnp, &dnp->dn_spill, &czb);
    }

    if (err == 0 && (td->td_flags & TRAVERSE_POST)) {
        SET_BOOKMARK(&czb, objset, object, ZB_DNODE_LEVEL,
            ZB_DNODE_BLKID);
        err = td->td_func(td->td_spa, NULL, NULL, &czb, dnp,
            td->td_arg);
        if (err == TRAVERSE_VISIT_NO_CHILDREN)
            return (0);
        if (err != 0)
            return (err);
    }
    return (err);
}

/* ARGSUSED */
static int
traverse_prefetcher(spa_t *spa, zilog_t *zilog, const blkptr_t *bp,
    const zbookmark_phys_t *zb, const dnode_phys_t *dnp, void *arg)
{
    prefetch_data_t *pfd = arg;
    arc_flags_t aflags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;

    ASSERT(pfd->pd_bytes_fetched >= 0);
    if (bp == NULL)
        return (0);
    if (pfd->pd_cancel)
        return (SET_ERROR(EINTR));

    if (!prefetch_needed(pfd, bp))
        return (0);

    mutex_enter(&pfd->pd_mtx);
    while (!pfd->pd_cancel && pfd->pd_bytes_fetched >= zfs_pd_bytes_max)
        cv_wait(&pfd->pd_cv, &pfd->pd_mtx);
    pfd->pd_bytes_fetched += BP_GET_LSIZE(bp);
    cv_broadcast(&pfd->pd_cv);
    mutex_exit(&pfd->pd_mtx);

    (void) arc_read(NULL, spa, bp, NULL, NULL, ZIO_PRIORITY_ASYNC_READ,
        ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE, &aflags, zb);

    return (0);
}

static void
traverse_prefetch_thread(void *arg)
{
    traverse_data_t *td_main = arg;
    traverse_data_t td = *td_main;
    zbookmark_phys_t czb;

    td.td_func = traverse_prefetcher;
    td.td_arg = td_main->td_pfd;
    td.td_pfd = NULL;
    td.td_resume = &td_main->td_pfd->pd_resume;

    SET_BOOKMARK(&czb, td.td_objset,
        ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
    (void) traverse_visitbp(&td, NULL, td.td_rootbp, &czb);

    mutex_enter(&td_main->td_pfd->pd_mtx);
    td_main->td_pfd->pd_exited = B_TRUE;
    cv_broadcast(&td_main->td_pfd->pd_cv);
    mutex_exit(&td_main->td_pfd->pd_mtx);
}

/*
 * NB: dataset must not be changing on-disk (eg, is a snapshot or we are
 * in syncing context).
 */
static int
traverse_impl(spa_t *spa, dsl_dataset_t *ds, uint64_t objset, blkptr_t *rootbp,
    uint64_t txg_start, zbookmark_phys_t *resume, int flags,
    blkptr_cb_t func, void *arg)
{
    traverse_data_t td;
    prefetch_data_t pd = { 0 };
    zbookmark_phys_t czb;
    int err;

    ASSERT(ds == NULL || objset == ds->ds_object);
    ASSERT(!(flags & TRAVERSE_PRE) || !(flags & TRAVERSE_POST));

    td.td_spa = spa;
    td.td_objset = objset;
    td.td_rootbp = rootbp;
    td.td_min_txg = txg_start;
    td.td_resume = resume;
    td.td_func = func;
    td.td_arg = arg;
    td.td_pfd = &pd;
    td.td_flags = flags;
    td.td_paused = B_FALSE;
    td.td_realloc_possible = (txg_start == 0 ? B_FALSE : B_TRUE);

    if (spa_feature_is_active(spa, SPA_FEATURE_HOLE_BIRTH)) {
        VERIFY(spa_feature_enabled_txg(spa,
            SPA_FEATURE_HOLE_BIRTH, &td.td_hole_birth_enabled_txg));
    } else {
        td.td_hole_birth_enabled_txg = UINT64_MAX;
    }

    pd.pd_flags = flags;
    if (resume != NULL)
        pd.pd_resume = *resume;
    mutex_init(&pd.pd_mtx, NULL, MUTEX_DEFAULT, NULL);
    cv_init(&pd.pd_cv, NULL, CV_DEFAULT, NULL);

    /* See comment on ZIL traversal in dsl_scan_visitds. */
    if (ds != NULL && !ds->ds_is_snapshot && !BP_IS_HOLE(rootbp)) {
        arc_flags_t flags = ARC_FLAG_WAIT;
        objset_phys_t *osp;
        arc_buf_t *buf;

        err = arc_read(NULL, td.td_spa, rootbp,
            arc_getbuf_func, &buf,
            ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, NULL);
        if (err != 0)
            return (err);

        osp = buf->b_data;
        traverse_zil(&td, &osp->os_zil_header);
        arc_buf_destroy(buf, &buf);
    }

    if (!(flags & TRAVERSE_PREFETCH_DATA) ||
        0 == taskq_dispatch(system_taskq, traverse_prefetch_thread,
        &td, TQ_NOQUEUE))
        pd.pd_exited = B_TRUE;

    SET_BOOKMARK(&czb, td.td_objset,
        ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
    err = traverse_visitbp(&td, NULL, rootbp, &czb);

    mutex_enter(&pd.pd_mtx);
    pd.pd_cancel = B_TRUE;
    cv_broadcast(&pd.pd_cv);
    while (!pd.pd_exited)
        cv_wait(&pd.pd_cv, &pd.pd_mtx);
    mutex_exit(&pd.pd_mtx);

    mutex_destroy(&pd.pd_mtx);
    cv_destroy(&pd.pd_cv);

    return (err);
}

/*
 * NB: dataset must not be changing on-disk (eg, is a snapshot or we are
 * in syncing context).
 */
int
traverse_dataset_resume(dsl_dataset_t *ds, uint64_t txg_start,
    zbookmark_phys_t *resume,
    int flags, blkptr_cb_t func, void *arg)
{
    return (traverse_impl(ds->ds_dir->dd_pool->dp_spa, ds, ds->ds_object,
        &dsl_dataset_phys(ds)->ds_bp, txg_start, resume, flags, func, arg));
}

int
traverse_dataset(dsl_dataset_t *ds, uint64_t txg_start,
    int flags, blkptr_cb_t func, void *arg)
{
    return (traverse_dataset_resume(ds, txg_start, NULL, flags, func, arg));
}

int
traverse_dataset_destroyed(spa_t *spa, blkptr_t *blkptr,
    uint64_t txg_start, zbookmark_phys_t *resume, int flags,
    blkptr_cb_t func, void *arg)
{
    return (traverse_impl(spa, NULL, ZB_DESTROYED_OBJSET,
        blkptr, txg_start, resume, flags, func, arg));
}

/*
 * NB: pool must not be changing on-disk (eg, from zdb or sync context).
 */
int
traverse_pool(spa_t *spa, uint64_t txg_start, int flags,
    blkptr_cb_t func, void *arg)
{
    int err;
    dsl_pool_t *dp = spa_get_dsl(spa);
    objset_t *mos = dp->dp_meta_objset;
    boolean_t hard = (flags & TRAVERSE_HARD);

    /* visit the MOS */
    err = traverse_impl(spa, NULL, 0, spa_get_rootblkptr(spa),
        txg_start, NULL, flags, func, arg);
    if (err != 0)
        return (err);

    /* visit each dataset */
    for (uint64_t obj = 1; err == 0;
        err = dmu_object_next(mos, &obj, B_FALSE, txg_start)) {
        dmu_object_info_t doi;

        err = dmu_object_info(mos, obj, &doi);
        if (err != 0) {
            if (hard)
                continue;
            break;
        }

        if (doi.doi_bonus_type == DMU_OT_DSL_DATASET) {
            dsl_dataset_t *ds;
            uint64_t txg = txg_start;

            dsl_pool_config_enter(dp, FTAG);
            err = dsl_dataset_hold_obj(dp, obj, FTAG, &ds);
            dsl_pool_config_exit(dp, FTAG);
            if (err != 0) {
                if (hard)
                    continue;
                break;
            }
            if (dsl_dataset_phys(ds)->ds_prev_snap_txg > txg)
                txg = dsl_dataset_phys(ds)->ds_prev_snap_txg;
            err = traverse_dataset(ds, txg, flags, func, arg);
            dsl_dataset_rele(ds, FTAG);
            if (err != 0)
                break;
        }
    }
    if (err == ESRCH)
        err = 0;
    return (err);
}