xref: /titanic_41/usr/src/uts/common/fs/zfs/dmu_traverse.c (revision 416a91e24418fb283022309d06e7d7ce0094d70e)

/*
 * 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, 2014 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 */

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;
} 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;
} 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 a
		 * hole created before the SPA_FEATURE_HOLE_BIRTH
		 * feature was enabled.  If SPA_FEATURE_HOLE_BIRTH
		 * was enabled before the min_txg for this traveral we
		 * know the hole must have been created before the
		 * min_txg for this traveral, so we can skip it. If
		 * SPA_FEATURE_HOLE_BIRTH was enabled after the min_txg
		 * for this traveral we cannot tell if the hole was
		 * created before or after the min_txg for this
		 * traversal, so we cannot skip it.
		 */
		if (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;
		dnp = buf->b_data;

		for (i = 0; i < epb; i++) {
			prefetch_dnode_metadata(td, &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, &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;
		objset_phys_t *osp;
		dnode_phys_t *dnp;

		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;

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

	if (buf)
		(void) arc_buf_remove_ref(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.
		 */
		td->td_resume->zb_blkid = zb->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 (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;

	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));

	/*
	 * The data prefetching mechanism (the prefetch thread) is incompatible
	 * with resuming from a bookmark.
	 */
	ASSERT(resume == NULL || !(flags & TRAVERSE_PREFETCH_DATA));

	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;

	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 = 0;
	}

	pd.pd_flags = flags;
	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);
		(void) arc_buf_remove_ref(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(dsl_dataset_t *ds, uint64_t txg_start, 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, 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;
	uint64_t obj;
	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 (obj = 1; err == 0;
	    err = dmu_object_next(mos, &obj, 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);
}