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Fix stalled txg with repeated noop scans
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Currently, the DSL scan code figures out when it should suspend
processing and allow a txg to continue by calling the function
dsl_scan_check_suspend(). Unfortunately, this function only
allows the scan to suspend at a level 0 block. In the event that
the system is scanning a bunch of empty snapshots or a resilver
is running with a high enough scn_cur_min_txg, the scan will
stop processing each dataset at the root level, deciding it
has nothing left to do. This means that the check_suspend
function is never called and the txg remains stuck until a
dataset is found that has data to scan.

This patch fixes the problem by allowing scans to suspend at
the root level of the objset. For backwards compatibility, we
use the bookmark <objsetid, 0, 0, 0> when we suspend here so
that older versions of the code will work as intended.

Reviewed-by: Matt Ahrens <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tom Caputi <[email protected]>
Closes openzfs#9300
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Tom Caputi authored and tonyhutter committed Dec 27, 2019
1 parent 546e443 commit 91e1543
Showing 1 changed file with 12 additions and 3 deletions.
15 changes: 12 additions & 3 deletions module/zfs/dsl_scan.c
Original file line number Diff line number Diff line change
Expand Up @@ -1275,8 +1275,8 @@ dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
if (!ZB_IS_ZERO(&scn->scn_phys.scn_bookmark))
return (B_FALSE); /* we're resuming */

/* We only know how to resume from level-0 blocks. */
if (zb && zb->zb_level != 0)
/* We only know how to resume from level-0 and objset blocks. */
if (zb && (zb->zb_level != 0 && zb->zb_level != ZB_ROOT_LEVEL))
return (B_FALSE);

/*
Expand Down Expand Up @@ -1307,7 +1307,16 @@ dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
NSEC2SEC(sync_time_ns) >= zfs_txg_timeout)) ||
spa_shutting_down(scn->scn_dp->dp_spa) ||
(zfs_scan_strict_mem_lim && dsl_scan_should_clear(scn))) {
if (zb) {
if (zb && zb->zb_level == ZB_ROOT_LEVEL) {
dprintf("suspending at first available bookmark "
"%llx/%llx/%llx/%llx\n",
(longlong_t)zb->zb_objset,
(longlong_t)zb->zb_object,
(longlong_t)zb->zb_level,
(longlong_t)zb->zb_blkid);
SET_BOOKMARK(&scn->scn_phys.scn_bookmark,
zb->zb_objset, 0, 0, 0);
} else if (zb != NULL) {
dprintf("suspending at bookmark %llx/%llx/%llx/%llx\n",
(longlong_t)zb->zb_objset,
(longlong_t)zb->zb_object,
Expand Down

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