spa_checkpoint.c revision 8671400134a11c848244896ca51a7db4d0f69da4
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(maptype_t type, uint64_t offset,
207    uint64_t size, void *arg)
208{
209	spa_checkpoint_discard_sync_callback_arg_t *sdc = arg;
210	vdev_t *vd = sdc->sdc_vd;
211	metaslab_t *ms = vd->vdev_ms[offset >> vd->vdev_ms_shift];
212	uint64_t end = offset + size;
213
214	if (sdc->sdc_entry_limit == 0)
215		return (EINTR);
216
217	/*
218	 * Since the space map is not condensed, we know that
219	 * none of its entries is crossing the boundaries of
220	 * its respective metaslab.
221	 *
222	 * That said, there is no fundamental requirement that
223	 * the checkpoint's space map entries should not cross
224	 * metaslab boundaries. So if needed we could add code
225	 * that handles metaslab-crossing segments in the future.
226	 */
227	VERIFY3U(type, ==, SM_FREE);
228	VERIFY3U(offset, >=, ms->ms_start);
229	VERIFY3U(end, <=, ms->ms_start + ms->ms_size);
230
231	/*
232	 * At this point we should not be processing any
233	 * other frees concurrently, so the lock is technically
234	 * unnecessary. We use the lock anyway though to
235	 * potentially save ourselves from future headaches.
236	 */
237	mutex_enter(&ms->ms_lock);
238	if (range_tree_is_empty(ms->ms_freeing))
239		vdev_dirty(vd, VDD_METASLAB, ms, sdc->sdc_txg);
240	range_tree_add(ms->ms_freeing, offset, size);
241	mutex_exit(&ms->ms_lock);
242
243	ASSERT3U(vd->vdev_spa->spa_checkpoint_info.sci_dspace, >=, size);
244	ASSERT3U(vd->vdev_stat.vs_checkpoint_space, >=, size);
245
246	vd->vdev_spa->spa_checkpoint_info.sci_dspace -= size;
247	vd->vdev_stat.vs_checkpoint_space -= size;
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	uint64_t max_entry_limit =
294	    (zfs_spa_discard_memory_limit / sizeof (uint64_t)) >> 1;
295
296	uint64_t entries_in_sm =
297	    space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t);
298
299	/*
300	 * Iterate from the end of the space map towards the beginning,
301	 * placing its entries on ms_freeing and removing them from the
302	 * space map. The iteration stops if one of the following
303	 * conditions is true:
304	 *
305	 * 1] We reached the beginning of the space map. At this point
306	 *    the space map should be completely empty and
307	 *    space_map_incremental_destroy should have returned 0.
308	 *    The next step would be to free and close the space map
309	 *    and remove its entry from its vdev's top zap. This allows
310	 *    spa_checkpoint_discard_thread() to move on to the next vdev.
311	 *
312	 * 2] We reached the memory limit (amount of memory used to hold
313	 *    space map entries in memory) and space_map_incremental_destroy
314	 *    returned EINTR. This means that there are entries remaining
315	 *    in the space map that will be cleared in a future invocation
316	 *    of this function by spa_checkpoint_discard_thread().
317	 */
318	spa_checkpoint_discard_sync_callback_arg_t sdc;
319	sdc.sdc_vd = vd;
320	sdc.sdc_txg = tx->tx_txg;
321	sdc.sdc_entry_limit = MIN(entries_in_sm, max_entry_limit);
322
323	uint64_t entries_before = entries_in_sm;
324
325	error = space_map_incremental_destroy(vd->vdev_checkpoint_sm,
326	    spa_checkpoint_discard_sync_callback, &sdc, tx);
327
328	uint64_t entries_after =
329	    space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t);
330
331#ifdef DEBUG
332	spa_checkpoint_accounting_verify(vd->vdev_spa);
333#endif
334
335	zfs_dbgmsg("discarding checkpoint: txg %llu, vdev id %d, "
336	    "deleted %llu entries - %llu entries are left",
337	    tx->tx_txg, vd->vdev_id, (entries_before - entries_after),
338	    entries_after);
339
340	if (error != EINTR) {
341		if (error != 0) {
342			zfs_panic_recover("zfs: error %d was returned "
343			    "while incrementally destroying the checkpoint "
344			    "space map of vdev %llu\n",
345			    error, vd->vdev_id);
346		}
347		ASSERT0(entries_after);
348		ASSERT0(vd->vdev_checkpoint_sm->sm_alloc);
349		ASSERT0(vd->vdev_checkpoint_sm->sm_length);
350
351		space_map_free(vd->vdev_checkpoint_sm, tx);
352		space_map_close(vd->vdev_checkpoint_sm);
353		vd->vdev_checkpoint_sm = NULL;
354
355		VERIFY0(zap_remove(vd->vdev_spa->spa_meta_objset,
356		    vd->vdev_top_zap, VDEV_TOP_ZAP_POOL_CHECKPOINT_SM, tx));
357	}
358}
359
360static boolean_t
361spa_checkpoint_discard_is_done(spa_t *spa)
362{
363	vdev_t *rvd = spa->spa_root_vdev;
364
365	ASSERT(!spa_has_checkpoint(spa));
366	ASSERT(spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT));
367
368	for (uint64_t c = 0; c < rvd->vdev_children; c++) {
369		if (rvd->vdev_child[c]->vdev_checkpoint_sm != NULL)
370			return (B_FALSE);
371		ASSERT0(rvd->vdev_child[c]->vdev_stat.vs_checkpoint_space);
372	}
373
374	return (B_TRUE);
375}
376
377/* ARGSUSED */
378boolean_t
379spa_checkpoint_discard_thread_check(void *arg, zthr_t *zthr)
380{
381	spa_t *spa = arg;
382
383	if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
384		return (B_FALSE);
385
386	if (spa_has_checkpoint(spa))
387		return (B_FALSE);
388
389	return (B_TRUE);
390}
391
392int
393spa_checkpoint_discard_thread(void *arg, zthr_t *zthr)
394{
395	spa_t *spa = arg;
396	vdev_t *rvd = spa->spa_root_vdev;
397
398	for (uint64_t c = 0; c < rvd->vdev_children; c++) {
399		vdev_t *vd = rvd->vdev_child[c];
400
401		while (vd->vdev_checkpoint_sm != NULL) {
402			space_map_t *checkpoint_sm = vd->vdev_checkpoint_sm;
403			int numbufs;
404			dmu_buf_t **dbp;
405
406			if (zthr_iscancelled(zthr))
407				return (0);
408
409			ASSERT3P(vd->vdev_ops, !=, &vdev_indirect_ops);
410
411			uint64_t size = MIN(space_map_length(checkpoint_sm),
412			    zfs_spa_discard_memory_limit);
413			uint64_t offset =
414			    space_map_length(checkpoint_sm) - size;
415
416			/*
417			 * Ensure that the part of the space map that will
418			 * be destroyed by the synctask, is prefetched in
419			 * memory before the synctask runs.
420			 */
421			int error = dmu_buf_hold_array_by_bonus(
422			    checkpoint_sm->sm_dbuf, offset, size,
423			    B_TRUE, FTAG, &numbufs, &dbp);
424			if (error != 0) {
425				zfs_panic_recover("zfs: error %d was returned "
426				    "while prefetching checkpoint space map "
427				    "entries of vdev %llu\n",
428				    error, vd->vdev_id);
429			}
430
431			VERIFY0(dsl_sync_task(spa->spa_name, NULL,
432			    spa_checkpoint_discard_thread_sync, vd,
433			    0, ZFS_SPACE_CHECK_NONE));
434
435			dmu_buf_rele_array(dbp, numbufs, FTAG);
436		}
437	}
438
439	VERIFY(spa_checkpoint_discard_is_done(spa));
440	VERIFY0(spa->spa_checkpoint_info.sci_dspace);
441	VERIFY0(dsl_sync_task(spa->spa_name, NULL,
442	    spa_checkpoint_discard_complete_sync, spa,
443	    0, ZFS_SPACE_CHECK_NONE));
444
445	return (0);
446}
447
448
449/* ARGSUSED */
450static int
451spa_checkpoint_check(void *arg, dmu_tx_t *tx)
452{
453	spa_t *spa = dmu_tx_pool(tx)->dp_spa;
454
455	if (!spa_feature_is_enabled(spa, SPA_FEATURE_POOL_CHECKPOINT))
456		return (SET_ERROR(ENOTSUP));
457
458	if (!spa_top_vdevs_spacemap_addressable(spa))
459		return (SET_ERROR(ZFS_ERR_VDEV_TOO_BIG));
460
461	if (spa->spa_vdev_removal != NULL)
462		return (SET_ERROR(ZFS_ERR_DEVRM_IN_PROGRESS));
463
464	if (spa->spa_checkpoint_txg != 0)
465		return (SET_ERROR(ZFS_ERR_CHECKPOINT_EXISTS));
466
467	if (spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
468		return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT));
469
470	return (0);
471}
472
473/* ARGSUSED */
474static void
475spa_checkpoint_sync(void *arg, dmu_tx_t *tx)
476{
477	dsl_pool_t *dp = dmu_tx_pool(tx);
478	spa_t *spa = dp->dp_spa;
479	uberblock_t checkpoint = spa->spa_ubsync;
480
481	/*
482	 * At this point, there should not be a checkpoint in the MOS.
483	 */
484	ASSERT3U(zap_contains(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
485	    DMU_POOL_ZPOOL_CHECKPOINT), ==, ENOENT);
486
487	ASSERT0(spa->spa_checkpoint_info.sci_timestamp);
488	ASSERT0(spa->spa_checkpoint_info.sci_dspace);
489
490	/*
491	 * Since the checkpointed uberblock is the one that just got synced
492	 * (we use spa_ubsync), its txg must be equal to the txg number of
493	 * the txg we are syncing, minus 1.
494	 */
495	ASSERT3U(checkpoint.ub_txg, ==, spa->spa_syncing_txg - 1);
496
497	/*
498	 * Once the checkpoint is in place, we need to ensure that none of
499	 * its blocks will be marked for reuse after it has been freed.
500	 * When there is a checkpoint and a block is freed, we compare its
501	 * birth txg to the txg of the checkpointed uberblock to see if the
502	 * block is part of the checkpoint or not. Therefore, we have to set
503	 * spa_checkpoint_txg before any frees happen in this txg (which is
504	 * why this is done as an early_synctask as explained in the comment
505	 * in spa_checkpoint()).
506	 */
507	spa->spa_checkpoint_txg = checkpoint.ub_txg;
508	spa->spa_checkpoint_info.sci_timestamp = checkpoint.ub_timestamp;
509
510	checkpoint.ub_checkpoint_txg = checkpoint.ub_txg;
511	VERIFY0(zap_add(spa->spa_dsl_pool->dp_meta_objset,
512	    DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT,
513	    sizeof (uint64_t), sizeof (uberblock_t) / sizeof (uint64_t),
514	    &checkpoint, tx));
515
516	/*
517	 * Increment the feature refcount and thus activate the feature.
518	 * Note that the feature will be deactivated when we've
519	 * completely discarded all checkpointed state (both vdev
520	 * space maps and uberblock).
521	 */
522	spa_feature_incr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx);
523
524	spa_history_log_internal(spa, "spa checkpoint", tx,
525	    "checkpointed uberblock txg=%llu", checkpoint.ub_txg);
526}
527
528/*
529 * Create a checkpoint for the pool.
530 */
531int
532spa_checkpoint(const char *pool)
533{
534	int error;
535	spa_t *spa;
536
537	error = spa_open(pool, &spa, FTAG);
538	if (error != 0)
539		return (error);
540
541	mutex_enter(&spa->spa_vdev_top_lock);
542
543	/*
544	 * Wait for current syncing txg to finish so the latest synced
545	 * uberblock (spa_ubsync) has all the changes that we expect
546	 * to see if we were to revert later to the checkpoint. In other
547	 * words we want the checkpointed uberblock to include/reference
548	 * all the changes that were pending at the time that we issued
549	 * the checkpoint command.
550	 */
551	txg_wait_synced(spa_get_dsl(spa), 0);
552
553	/*
554	 * As the checkpointed uberblock references blocks from the previous
555	 * txg (spa_ubsync) we want to ensure that are not freeing any of
556	 * these blocks in the same txg that the following synctask will
557	 * run. Thus, we run it as an early synctask, so the dirty changes
558	 * that are synced to disk afterwards during zios and other synctasks
559	 * do not reuse checkpointed blocks.
560	 */
561	error = dsl_early_sync_task(pool, spa_checkpoint_check,
562	    spa_checkpoint_sync, NULL, 0, ZFS_SPACE_CHECK_NORMAL);
563
564	mutex_exit(&spa->spa_vdev_top_lock);
565
566	spa_close(spa, FTAG);
567	return (error);
568}
569
570/* ARGSUSED */
571static int
572spa_checkpoint_discard_check(void *arg, dmu_tx_t *tx)
573{
574	spa_t *spa = dmu_tx_pool(tx)->dp_spa;
575
576	if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
577		return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT));
578
579	if (spa->spa_checkpoint_txg == 0)
580		return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT));
581
582	VERIFY0(zap_contains(spa_meta_objset(spa),
583	    DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT));
584
585	return (0);
586}
587
588/* ARGSUSED */
589static void
590spa_checkpoint_discard_sync(void *arg, dmu_tx_t *tx)
591{
592	spa_t *spa = dmu_tx_pool(tx)->dp_spa;
593
594	VERIFY0(zap_remove(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
595	    DMU_POOL_ZPOOL_CHECKPOINT, tx));
596
597	spa->spa_checkpoint_txg = 0;
598
599	zthr_wakeup(spa->spa_checkpoint_discard_zthr);
600
601	spa_history_log_internal(spa, "spa discard checkpoint", tx,
602	    "started discarding checkpointed state from the pool");
603}
604
605/*
606 * Discard the checkpoint from a pool.
607 */
608int
609spa_checkpoint_discard(const char *pool)
610{
611	/*
612	 * Similarly to spa_checkpoint(), we want our synctask to run
613	 * before any pending dirty data are written to disk so they
614	 * won't end up in the checkpoint's data structures (e.g.
615	 * ms_checkpointing and vdev_checkpoint_sm) and re-create any
616	 * space maps that the discarding open-context thread has
617	 * deleted.
618	 * [see spa_discard_checkpoint_sync and spa_discard_checkpoint_thread]
619	 */
620	return (dsl_early_sync_task(pool, spa_checkpoint_discard_check,
621	    spa_checkpoint_discard_sync, NULL, 0,
622	    ZFS_SPACE_CHECK_DISCARD_CHECKPOINT));
623}
624