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Defer async destroys on pool import

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We've observed a number of cases when pool import stuck for many
minutes due to large async destroy trying to load DDT or BRT from
HDD pool.  While proper destroy dosage is a separate problem,
lets give import process a chance to complete before that at all.
It may be not enough if there is a lot of ZIL to replay, but that
is harder to cover, since those are in separate syscalls.

Code investigation shown that we already have this mechanism used
for scrub/resilver, so this patch converts SCAN_IMPORT_WAIT_TXGS
into a tunable and applies it to async destroys also.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #18033
This commit is contained in:
Alexander Motin 2025-12-09 15:16:46 -05:00 committed by Brian Behlendorf
parent d976587a35
commit f72fd378c8
2 changed files with 28 additions and 16 deletions
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10
man/man4/zfs.4
View File

@ -1767,6 +1767,16 @@ Blocks that go to the special vdevs are still written indirectly, as with
.Sy logbias Ns = Ns Sy throughput .
This parameter is ignored if an SLOG is present.
.
.It Sy zfs_import_defer_txgs Ns = Ns Sy 5 Pq uint
Number of transaction groups to wait after pool import before starting
background work such as asynchronous block freeing
.Pq from snapshots, clones, and deduplication
and scrub or resilver operations.
This allows the pool import and filesystem mounting to complete more quickly
without interference from background activities.
The default value of 5 transaction groups typically provides sufficient time
for import and mount operations to complete on most systems.
.
.It Sy zfs_initialize_value Ns = Ns Sy 16045690984833335022 Po 0xDEADBEEFDEADBEEE Pc Pq u64
Pattern written to vdev free space by
.Xr zpool-initialize 8 .

34
module/zfs/dsl_scan.c
View File

@ -217,16 +217,14 @@ static int zfs_resilver_disable_defer = B_FALSE;
static uint_t zfs_resilver_defer_percent = 10;
/*
* We wait a few txgs after importing a pool to begin scanning so that
* the import / mounting code isn't held up by scrub / resilver IO.
* Unfortunately, it is a bit difficult to determine exactly how long
* this will take since userspace will trigger fs mounts asynchronously
* and the kernel will create zvol minors asynchronously. As a result,
* the value provided here is a bit arbitrary, but represents a
* reasonable estimate of how many txgs it will take to finish fully
* importing a pool
* Number of TXGs to wait after importing before starting background
* work (async destroys, scan/scrub/resilver operations). This allows
* the import command and filesystem mounts to complete quickly without
* being delayed by background activities. The value is somewhat arbitrary
* since userspace triggers filesystem mounts asynchronously, but 5 TXGs
* provides a reasonable window for import completion in most cases.
*/
#define SCAN_IMPORT_WAIT_TXGS 5
static uint_t zfs_import_defer_txgs = 5;
#define DSL_SCAN_IS_SCRUB_RESILVER(scn) \
((scn)->scn_phys.scn_func == POOL_SCAN_SCRUB || \
@ -4394,6 +4392,14 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
if (spa_shutting_down(spa))
return;
/*
* Wait a few txgs after importing before doing background work
* (async destroys and scanning). This should help the import
* command to complete quickly.
*/
if (spa->spa_syncing_txg < spa->spa_first_txg + zfs_import_defer_txgs)
return;
/*
* If the scan is inactive due to a stalled async destroy, try again.
*/
@ -4430,13 +4436,6 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
if (!dsl_scan_is_running(scn) || dsl_scan_is_paused_scrub(scn))
return;
/*
* Wait a few txgs after importing to begin scanning so that
* we can get the pool imported quickly.
*/
if (spa->spa_syncing_txg < spa->spa_first_txg + SCAN_IMPORT_WAIT_TXGS)
return;
/*
* zfs_scan_suspend_progress can be set to disable scan progress.
* We don't want to spin the txg_sync thread, so we add a delay
@ -5336,6 +5335,9 @@ ZFS_MODULE_PARAM(zfs, zfs_, scan_issue_strategy, UINT, ZMOD_RW,
ZFS_MODULE_PARAM(zfs, zfs_, scan_legacy, INT, ZMOD_RW,
"Scrub using legacy non-sequential method");
ZFS_MODULE_PARAM(zfs, zfs_, import_defer_txgs, UINT, ZMOD_RW,
"Number of TXGs to defer background work after pool import");
ZFS_MODULE_PARAM(zfs, zfs_, scan_checkpoint_intval, UINT, ZMOD_RW,
"Scan progress on-disk checkpointing interval");