spa_checkpoint.c revision 17f11284b49b98353b5119463254074fd9bc0a28
1/*
2 * CDDL HEADER START
3 *
4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License (the "License").
6 * You may not use this file except in compliance with the License.
7 *
8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9 * or http://www.opensolaris.org/os/licensing.
10 * See the License for the specific language governing permissions
11 * and limitations under the License.
12 *
13 * When distributing Covered Code, include this CDDL HEADER in each
14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15 * If applicable, add the following below this CDDL HEADER, with the
16 * fields enclosed by brackets "[]" replaced with your own identifying
17 * information: Portions Copyright [yyyy] [name of copyright owner]
18 *
19 * CDDL HEADER END
20 */
21
22/*
23 * Copyright (c) 2017 by Delphix. All rights reserved.
24 */
25
26/*
27 * Storage Pool Checkpoint
28 *
29 * A storage pool checkpoint can be thought of as a pool-wide snapshot or
30 * a stable version of extreme rewind that guarantees no blocks from the
31 * checkpointed state will have been overwritten. It remembers the entire
32 * state of the storage pool (e.g. snapshots, dataset names, etc..) from the
33 * point that it was taken and the user can rewind back to that point even if
34 * they applied destructive operations on their datasets or even enabled new
35 * zpool on-disk features. If a pool has a checkpoint that is no longer
36 * needed, the user can discard it.
37 *
38 * == On disk data structures used ==
39 *
40 * - The pool has a new feature flag and a new entry in the MOS. The feature
41 *   flag is set to active when we create the checkpoint and remains active
42 *   until the checkpoint is fully discarded. The entry in the MOS config
43 *   (DMU_POOL_ZPOOL_CHECKPOINT) is populated with the uberblock that
44 *   references the state of the pool when we take the checkpoint. The entry
45 *   remains populated until we start discarding the checkpoint or we rewind
46 *   back to it.
47 *
48 * - Each vdev contains a vdev-wide space map while the pool has a checkpoint,
49 *   which persists until the checkpoint is fully discarded. The space map
50 *   contains entries that have been freed in the current state of the pool
51 *   but we want to keep around in case we decide to rewind to the checkpoint.
52 *   [see vdev_checkpoint_sm]
53 *
54 * - Each metaslab's ms_sm space map behaves the same as without the
55 *   checkpoint, with the only exception being the scenario when we free
56 *   blocks that belong to the checkpoint. In this case, these blocks remain
57 *   ALLOCATED in the metaslab's space map and they are added as FREE in the
58 *   vdev's checkpoint space map.
59 *
60 * - Each uberblock has a field (ub_checkpoint_txg) which holds the txg that
61 *   the uberblock was checkpointed. For normal uberblocks this field is 0.
62 *
63 * == Overview of operations ==
64 *
65 * - To create a checkpoint, we first wait for the current TXG to be synced,
66 *   so we can use the most recently synced uberblock (spa_ubsync) as the
67 *   checkpointed uberblock. Then we use an early synctask to place that
68 *   uberblock in MOS config, increment the feature flag for the checkpoint
69 *   (marking it active), and setting spa_checkpoint_txg (see its use below)
70 *   to the TXG of the checkpointed uberblock. We use an early synctask for
71 *   the aforementioned operations to ensure that no blocks were dirtied
72 *   between the current TXG and the TXG of the checkpointed uberblock
73 *   (e.g the previous txg).
74 *
75 * - When a checkpoint exists, we need to ensure that the blocks that
76 *   belong to the checkpoint are freed but never reused. This means that
77 *   these blocks should never end up in the ms_allocatable or the ms_freeing
78 *   trees of a metaslab. Therefore, whenever there is a checkpoint the new
79 *   ms_checkpointing tree is used in addition to the aforementioned ones.
80 *
81 *   Whenever a block is freed and we find out that it is referenced by the
82 *   checkpoint (we find out by comparing its birth to spa_checkpoint_txg),
83 *   we place it in the ms_checkpointing tree instead of the ms_freeingtree.
84 *   This way, we divide the blocks that are being freed into checkpointed
85 *   and not-checkpointed blocks.
86 *
87 *   In order to persist these frees, we write the extents from the
88 *   ms_freeingtree to the ms_sm as usual, and the extents from the
89 *   ms_checkpointing tree to the vdev_checkpoint_sm. This way, these
90 *   checkpointed extents will remain allocated in the metaslab's ms_sm space
91 *   map, and therefore won't be reused [see metaslab_sync()]. In addition,
92 *   when we discard the checkpoint, we can find the entries that have
93 *   actually been freed in vdev_checkpoint_sm.
94 *   [see spa_checkpoint_discard_thread_sync()]
95 *
96 * - To discard the checkpoint we use an early synctask to delete the
97 *   checkpointed uberblock from the MOS config, set spa_checkpoint_txg to 0,
98 *   and wakeup the discarding zthr thread (an open-context async thread).
99 *   We use an early synctask to ensure that the operation happens before any
100 *   new data end up in the checkpoint's data structures.
101 *
102 *   Once the synctask is done and the discarding zthr is awake, we discard
103 *   the checkpointed data over multiple TXGs by having the zthr prefetching
104 *   entries from vdev_checkpoint_sm and then starting a synctask that places
105 *   them as free blocks in to their respective ms_allocatable and ms_sm
106 *   structures.
107 *   [see spa_checkpoint_discard_thread()]
108 *
109 *   When there are no entries left in the vdev_checkpoint_sm of all
110 *   top-level vdevs, a final synctask runs that decrements the feature flag.
111 *
112 * - To rewind to the checkpoint, we first use the current uberblock and
113 *   open the MOS so we can access the checkpointed uberblock from the MOS
114 *   config. After we retrieve the checkpointed uberblock, we use it as the
115 *   current uberblock for the pool by writing it to disk with an updated
116 *   TXG, opening its version of the MOS, and moving on as usual from there.
117 *   [see spa_ld_checkpoint_rewind()]
118 *
119 *   An important note on rewinding to the checkpoint has to do with how we
120 *   handle ZIL blocks. In the scenario of a rewind, we clear out any ZIL
121 *   blocks that have not been claimed by the time we took the checkpoint
122 *   as they should no longer be valid.
123 *   [see comment in zil_claim()]
124 *
125 * == Miscellaneous information ==
126 *
127 * - In the hypothetical event that we take a checkpoint, remove a vdev,
128 *   and attempt to rewind, the rewind would fail as the checkpointed
129 *   uberblock would reference data in the removed device. For this reason
130 *   and others of similar nature, we disallow the following operations that
131 *   can change the config:
132 *   	vdev removal and attach/detach, mirror splitting, and pool reguid.
133 *
134 * - As most of the checkpoint logic is implemented in the SPA and doesn't
135 *   distinguish datasets when it comes to space accounting, having a
136 *   checkpoint can potentially break the boundaries set by dataset
137 *   reservations.
138 */
139
140#include <sys/dmu_tx.h>
141#include <sys/dsl_dir.h>
142#include <sys/dsl_synctask.h>
143#include <sys/metaslab_impl.h>
144#include <sys/spa.h>
145#include <sys/spa_impl.h>
146#include <sys/spa_checkpoint.h>
147#include <sys/vdev_impl.h>
148#include <sys/zap.h>
149#include <sys/zfeature.h>
150
151/*
152 * The following parameter limits the amount of memory to be used for the
153 * prefetching of the checkpoint space map done on each vdev while
154 * discarding the checkpoint.
155 *
156 * The reason it exists is because top-level vdevs with long checkpoint
157 * space maps can potentially take up a lot of memory depending on the
158 * amount of checkpointed data that has been freed within them while
159 * the pool had a checkpoint.
160 */
161uint64_t	zfs_spa_discard_memory_limit = 16 * 1024 * 1024;
162
163int
164spa_checkpoint_get_stats(spa_t *spa, pool_checkpoint_stat_t *pcs)
165{
166	if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
167		return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT));
168
169	bzero(pcs, sizeof (pool_checkpoint_stat_t));
170
171	int error = zap_contains(spa_meta_objset(spa),
172	    DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT);
173	ASSERT(error == 0 || error == ENOENT);
174
175	if (error == ENOENT)
176		pcs->pcs_state = CS_CHECKPOINT_DISCARDING;
177	else
178		pcs->pcs_state = CS_CHECKPOINT_EXISTS;
179
180	pcs->pcs_space = spa->spa_checkpoint_info.sci_dspace;
181	pcs->pcs_start_time = spa->spa_checkpoint_info.sci_timestamp;
182
183	return (0);
184}
185
186static void
187spa_checkpoint_discard_complete_sync(void *arg, dmu_tx_t *tx)
188{
189	spa_t *spa = arg;
190
191	spa->spa_checkpoint_info.sci_timestamp = 0;
192
193	spa_feature_decr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx);
194
195	spa_history_log_internal(spa, "spa discard checkpoint", tx,
196	    "finished discarding checkpointed state from the pool");
197}
198
199typedef struct spa_checkpoint_discard_sync_callback_arg {
200	vdev_t *sdc_vd;
201	uint64_t sdc_txg;
202	uint64_t sdc_entry_limit;
203} spa_checkpoint_discard_sync_callback_arg_t;
204
205static int
206spa_checkpoint_discard_sync_callback(space_map_entry_t *sme, void *arg)
207{
208	spa_checkpoint_discard_sync_callback_arg_t *sdc = arg;
209	vdev_t *vd = sdc->sdc_vd;
210	metaslab_t *ms = vd->vdev_ms[sme->sme_offset >> vd->vdev_ms_shift];
211	uint64_t end = sme->sme_offset + sme->sme_run;
212
213	if (sdc->sdc_entry_limit == 0)
214		return (EINTR);
215
216	/*
217	 * Since the space map is not condensed, we know that
218	 * none of its entries is crossing the boundaries of
219	 * its respective metaslab.
220	 *
221	 * That said, there is no fundamental requirement that
222	 * the checkpoint's space map entries should not cross
223	 * metaslab boundaries. So if needed we could add code
224	 * that handles metaslab-crossing segments in the future.
225	 */
226	VERIFY3U(sme->sme_type, ==, SM_FREE);
227	VERIFY3U(sme->sme_offset, >=, ms->ms_start);
228	VERIFY3U(end, <=, ms->ms_start + ms->ms_size);
229
230	/*
231	 * At this point we should not be processing any
232	 * other frees concurrently, so the lock is technically
233	 * unnecessary. We use the lock anyway though to
234	 * potentially save ourselves from future headaches.
235	 */
236	mutex_enter(&ms->ms_lock);
237	if (range_tree_is_empty(ms->ms_freeing))
238		vdev_dirty(vd, VDD_METASLAB, ms, sdc->sdc_txg);
239	range_tree_add(ms->ms_freeing, sme->sme_offset, sme->sme_run);
240	mutex_exit(&ms->ms_lock);
241
242	ASSERT3U(vd->vdev_spa->spa_checkpoint_info.sci_dspace, >=,
243	    sme->sme_run);
244	ASSERT3U(vd->vdev_stat.vs_checkpoint_space, >=, sme->sme_run);
245
246	vd->vdev_spa->spa_checkpoint_info.sci_dspace -= sme->sme_run;
247	vd->vdev_stat.vs_checkpoint_space -= sme->sme_run;
248	sdc->sdc_entry_limit--;
249
250	return (0);
251}
252
253static void
254spa_checkpoint_accounting_verify(spa_t *spa)
255{
256	vdev_t *rvd = spa->spa_root_vdev;
257	uint64_t ckpoint_sm_space_sum = 0;
258	uint64_t vs_ckpoint_space_sum = 0;
259
260	for (uint64_t c = 0; c < rvd->vdev_children; c++) {
261		vdev_t *vd = rvd->vdev_child[c];
262
263		if (vd->vdev_checkpoint_sm != NULL) {
264			ckpoint_sm_space_sum +=
265			    -vd->vdev_checkpoint_sm->sm_alloc;
266			vs_ckpoint_space_sum +=
267			    vd->vdev_stat.vs_checkpoint_space;
268			ASSERT3U(ckpoint_sm_space_sum, ==,
269			    vs_ckpoint_space_sum);
270		} else {
271			ASSERT0(vd->vdev_stat.vs_checkpoint_space);
272		}
273	}
274	ASSERT3U(spa->spa_checkpoint_info.sci_dspace, ==, ckpoint_sm_space_sum);
275}
276
277static void
278spa_checkpoint_discard_thread_sync(void *arg, dmu_tx_t *tx)
279{
280	vdev_t *vd = arg;
281	int error;
282
283	/*
284	 * The space map callback is applied only to non-debug entries.
285	 * Because the number of debug entries is less or equal to the
286	 * number of non-debug entries, we want to ensure that we only
287	 * read what we prefetched from open-context.
288	 *
289	 * Thus, we set the maximum entries that the space map callback
290	 * will be applied to be half the entries that could fit in the
291	 * imposed memory limit.
292	 *
293	 * Note that since this is a conservative estimate we also
294	 * assume the worst case scenario in our computation where each
295	 * entry is two-word.
296	 */
297	uint64_t max_entry_limit =
298	    (zfs_spa_discard_memory_limit / (2 * sizeof (uint64_t))) >> 1;
299
300	/*
301	 * Iterate from the end of the space map towards the beginning,
302	 * placing its entries on ms_freeing and removing them from the
303	 * space map. The iteration stops if one of the following
304	 * conditions is true:
305	 *
306	 * 1] We reached the beginning of the space map. At this point
307	 *    the space map should be completely empty and
308	 *    space_map_incremental_destroy should have returned 0.
309	 *    The next step would be to free and close the space map
310	 *    and remove its entry from its vdev's top zap. This allows
311	 *    spa_checkpoint_discard_thread() to move on to the next vdev.
312	 *
313	 * 2] We reached the memory limit (amount of memory used to hold
314	 *    space map entries in memory) and space_map_incremental_destroy
315	 *    returned EINTR. This means that there are entries remaining
316	 *    in the space map that will be cleared in a future invocation
317	 *    of this function by spa_checkpoint_discard_thread().
318	 */
319	spa_checkpoint_discard_sync_callback_arg_t sdc;
320	sdc.sdc_vd = vd;
321	sdc.sdc_txg = tx->tx_txg;
322	sdc.sdc_entry_limit = max_entry_limit;
323
324	uint64_t words_before =
325	    space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t);
326
327	error = space_map_incremental_destroy(vd->vdev_checkpoint_sm,
328	    spa_checkpoint_discard_sync_callback, &sdc, tx);
329
330	uint64_t words_after =
331	    space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t);
332
333#ifdef DEBUG
334	spa_checkpoint_accounting_verify(vd->vdev_spa);
335#endif
336
337	zfs_dbgmsg("discarding checkpoint: txg %llu, vdev id %d, "
338	    "deleted %llu words - %llu words are left",
339	    tx->tx_txg, vd->vdev_id, (words_before - words_after),
340	    words_after);
341
342	if (error != EINTR) {
343		if (error != 0) {
344			zfs_panic_recover("zfs: error %d was returned "
345			    "while incrementally destroying the checkpoint "
346			    "space map of vdev %llu\n",
347			    error, vd->vdev_id);
348		}
349		ASSERT0(words_after);
350		ASSERT0(vd->vdev_checkpoint_sm->sm_alloc);
351		ASSERT0(space_map_length(vd->vdev_checkpoint_sm));
352
353		space_map_free(vd->vdev_checkpoint_sm, tx);
354		space_map_close(vd->vdev_checkpoint_sm);
355		vd->vdev_checkpoint_sm = NULL;
356
357		VERIFY0(zap_remove(spa_meta_objset(vd->vdev_spa),
358		    vd->vdev_top_zap, VDEV_TOP_ZAP_POOL_CHECKPOINT_SM, tx));
359	}
360}
361
362static boolean_t
363spa_checkpoint_discard_is_done(spa_t *spa)
364{
365	vdev_t *rvd = spa->spa_root_vdev;
366
367	ASSERT(!spa_has_checkpoint(spa));
368	ASSERT(spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT));
369
370	for (uint64_t c = 0; c < rvd->vdev_children; c++) {
371		if (rvd->vdev_child[c]->vdev_checkpoint_sm != NULL)
372			return (B_FALSE);
373		ASSERT0(rvd->vdev_child[c]->vdev_stat.vs_checkpoint_space);
374	}
375
376	return (B_TRUE);
377}
378
379/* ARGSUSED */
380boolean_t
381spa_checkpoint_discard_thread_check(void *arg, zthr_t *zthr)
382{
383	spa_t *spa = arg;
384
385	if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
386		return (B_FALSE);
387
388	if (spa_has_checkpoint(spa))
389		return (B_FALSE);
390
391	return (B_TRUE);
392}
393
394int
395spa_checkpoint_discard_thread(void *arg, zthr_t *zthr)
396{
397	spa_t *spa = arg;
398	vdev_t *rvd = spa->spa_root_vdev;
399
400	for (uint64_t c = 0; c < rvd->vdev_children; c++) {
401		vdev_t *vd = rvd->vdev_child[c];
402
403		while (vd->vdev_checkpoint_sm != NULL) {
404			space_map_t *checkpoint_sm = vd->vdev_checkpoint_sm;
405			int numbufs;
406			dmu_buf_t **dbp;
407
408			if (zthr_iscancelled(zthr))
409				return (0);
410
411			ASSERT3P(vd->vdev_ops, !=, &vdev_indirect_ops);
412
413			uint64_t size = MIN(space_map_length(checkpoint_sm),
414			    zfs_spa_discard_memory_limit);
415			uint64_t offset =
416			    space_map_length(checkpoint_sm) - size;
417
418			/*
419			 * Ensure that the part of the space map that will
420			 * be destroyed by the synctask, is prefetched in
421			 * memory before the synctask runs.
422			 */
423			int error = dmu_buf_hold_array_by_bonus(
424			    checkpoint_sm->sm_dbuf, offset, size,
425			    B_TRUE, FTAG, &numbufs, &dbp);
426			if (error != 0) {
427				zfs_panic_recover("zfs: error %d was returned "
428				    "while prefetching checkpoint space map "
429				    "entries of vdev %llu\n",
430				    error, vd->vdev_id);
431			}
432
433			VERIFY0(dsl_sync_task(spa->spa_name, NULL,
434			    spa_checkpoint_discard_thread_sync, vd,
435			    0, ZFS_SPACE_CHECK_NONE));
436
437			dmu_buf_rele_array(dbp, numbufs, FTAG);
438		}
439	}
440
441	VERIFY(spa_checkpoint_discard_is_done(spa));
442	VERIFY0(spa->spa_checkpoint_info.sci_dspace);
443	VERIFY0(dsl_sync_task(spa->spa_name, NULL,
444	    spa_checkpoint_discard_complete_sync, spa,
445	    0, ZFS_SPACE_CHECK_NONE));
446
447	return (0);
448}
449
450
451/* ARGSUSED */
452static int
453spa_checkpoint_check(void *arg, dmu_tx_t *tx)
454{
455	spa_t *spa = dmu_tx_pool(tx)->dp_spa;
456
457	if (!spa_feature_is_enabled(spa, SPA_FEATURE_POOL_CHECKPOINT))
458		return (SET_ERROR(ENOTSUP));
459
460	if (!spa_top_vdevs_spacemap_addressable(spa))
461		return (SET_ERROR(ZFS_ERR_VDEV_TOO_BIG));
462
463	if (spa->spa_vdev_removal != NULL)
464		return (SET_ERROR(ZFS_ERR_DEVRM_IN_PROGRESS));
465
466	if (spa->spa_checkpoint_txg != 0)
467		return (SET_ERROR(ZFS_ERR_CHECKPOINT_EXISTS));
468
469	if (spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
470		return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT));
471
472	return (0);
473}
474
475/* ARGSUSED */
476static void
477spa_checkpoint_sync(void *arg, dmu_tx_t *tx)
478{
479	dsl_pool_t *dp = dmu_tx_pool(tx);
480	spa_t *spa = dp->dp_spa;
481	uberblock_t checkpoint = spa->spa_ubsync;
482
483	/*
484	 * At this point, there should not be a checkpoint in the MOS.
485	 */
486	ASSERT3U(zap_contains(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
487	    DMU_POOL_ZPOOL_CHECKPOINT), ==, ENOENT);
488
489	ASSERT0(spa->spa_checkpoint_info.sci_timestamp);
490	ASSERT0(spa->spa_checkpoint_info.sci_dspace);
491
492	/*
493	 * Since the checkpointed uberblock is the one that just got synced
494	 * (we use spa_ubsync), its txg must be equal to the txg number of
495	 * the txg we are syncing, minus 1.
496	 */
497	ASSERT3U(checkpoint.ub_txg, ==, spa->spa_syncing_txg - 1);
498
499	/*
500	 * Once the checkpoint is in place, we need to ensure that none of
501	 * its blocks will be marked for reuse after it has been freed.
502	 * When there is a checkpoint and a block is freed, we compare its
503	 * birth txg to the txg of the checkpointed uberblock to see if the
504	 * block is part of the checkpoint or not. Therefore, we have to set
505	 * spa_checkpoint_txg before any frees happen in this txg (which is
506	 * why this is done as an early_synctask as explained in the comment
507	 * in spa_checkpoint()).
508	 */
509	spa->spa_checkpoint_txg = checkpoint.ub_txg;
510	spa->spa_checkpoint_info.sci_timestamp = checkpoint.ub_timestamp;
511
512	checkpoint.ub_checkpoint_txg = checkpoint.ub_txg;
513	VERIFY0(zap_add(spa->spa_dsl_pool->dp_meta_objset,
514	    DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT,
515	    sizeof (uint64_t), sizeof (uberblock_t) / sizeof (uint64_t),
516	    &checkpoint, tx));
517
518	/*
519	 * Increment the feature refcount and thus activate the feature.
520	 * Note that the feature will be deactivated when we've
521	 * completely discarded all checkpointed state (both vdev
522	 * space maps and uberblock).
523	 */
524	spa_feature_incr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx);
525
526	spa_history_log_internal(spa, "spa checkpoint", tx,
527	    "checkpointed uberblock txg=%llu", checkpoint.ub_txg);
528}
529
530/*
531 * Create a checkpoint for the pool.
532 */
533int
534spa_checkpoint(const char *pool)
535{
536	int error;
537	spa_t *spa;
538
539	error = spa_open(pool, &spa, FTAG);
540	if (error != 0)
541		return (error);
542
543	mutex_enter(&spa->spa_vdev_top_lock);
544
545	/*
546	 * Wait for current syncing txg to finish so the latest synced
547	 * uberblock (spa_ubsync) has all the changes that we expect
548	 * to see if we were to revert later to the checkpoint. In other
549	 * words we want the checkpointed uberblock to include/reference
550	 * all the changes that were pending at the time that we issued
551	 * the checkpoint command.
552	 */
553	txg_wait_synced(spa_get_dsl(spa), 0);
554
555	/*
556	 * As the checkpointed uberblock references blocks from the previous
557	 * txg (spa_ubsync) we want to ensure that are not freeing any of
558	 * these blocks in the same txg that the following synctask will
559	 * run. Thus, we run it as an early synctask, so the dirty changes
560	 * that are synced to disk afterwards during zios and other synctasks
561	 * do not reuse checkpointed blocks.
562	 */
563	error = dsl_early_sync_task(pool, spa_checkpoint_check,
564	    spa_checkpoint_sync, NULL, 0, ZFS_SPACE_CHECK_NORMAL);
565
566	mutex_exit(&spa->spa_vdev_top_lock);
567
568	spa_close(spa, FTAG);
569	return (error);
570}
571
572/* ARGSUSED */
573static int
574spa_checkpoint_discard_check(void *arg, dmu_tx_t *tx)
575{
576	spa_t *spa = dmu_tx_pool(tx)->dp_spa;
577
578	if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
579		return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT));
580
581	if (spa->spa_checkpoint_txg == 0)
582		return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT));
583
584	VERIFY0(zap_contains(spa_meta_objset(spa),
585	    DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT));
586
587	return (0);
588}
589
590/* ARGSUSED */
591static void
592spa_checkpoint_discard_sync(void *arg, dmu_tx_t *tx)
593{
594	spa_t *spa = dmu_tx_pool(tx)->dp_spa;
595
596	VERIFY0(zap_remove(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
597	    DMU_POOL_ZPOOL_CHECKPOINT, tx));
598
599	spa->spa_checkpoint_txg = 0;
600
601	zthr_wakeup(spa->spa_checkpoint_discard_zthr);
602
603	spa_history_log_internal(spa, "spa discard checkpoint", tx,
604	    "started discarding checkpointed state from the pool");
605}
606
607/*
608 * Discard the checkpoint from a pool.
609 */
610int
611spa_checkpoint_discard(const char *pool)
612{
613	/*
614	 * Similarly to spa_checkpoint(), we want our synctask to run
615	 * before any pending dirty data are written to disk so they
616	 * won't end up in the checkpoint's data structures (e.g.
617	 * ms_checkpointing and vdev_checkpoint_sm) and re-create any
618	 * space maps that the discarding open-context thread has
619	 * deleted.
620	 * [see spa_discard_checkpoint_sync and spa_discard_checkpoint_thread]
621	 */
622	return (dsl_early_sync_task(pool, spa_checkpoint_discard_check,
623	    spa_checkpoint_discard_sync, NULL, 0,
624	    ZFS_SPACE_CHECK_DISCARD_CHECKPOINT));
625}
626