fs/zfs/dsl_pool.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.
 */

#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/dsl_scan.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>

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;

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);

	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;

	err = dsl_scan_init(dp, txg);

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);
	dsl_scan_fini(dp);
	rw_destroy(&dp->dp_config_rwlock);
	mutex_destroy(&dp->dp_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);

	/* Initialize scan structures */
	VERIFY3U(0, ==, dsl_scan_init(dp, txg));

	/* 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;

	/*
	 * We need to copy dp_space_towrite() before doing
	 * dsl_sync_task_group_sync(), because
	 * dsl_dataset_snapshot_reserve_space() will increase
	 * dp_space_towrite but not actually write anything.
	 */
	data_written = dp->dp_space_towrite[txg & TXG_MASK];

	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
	 * userspace 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;

	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);

	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, 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));
}