Resilver restarts unnecessarily when it encounters errors

When a resilver finishes, vdev_dtl_reassess is called to hopefully
excise DTL_MISSING (amongst other things). If there are errors during
the resilver, they are tracked in DTL_SCRUB, as spelled out in the
block comment in vdev.c. DTL_SCRUB is in-core only, so it can only
be used if the pool was online for the whole resilver. This state is
tracked with the spa_scrub_started flag, which only gets set when
the scan is initialized. Unfortunately, this flag gets cleared right
before vdev_dtl_reassess gets called, so if there are any errors
during the scan, DTL_MISSING will never get excised and the resilver
will just continually restart. This fix simply moves clearing that
flag until after the call to vdev_dtl_reasses.

In addition, if a pool is imported and already has scn_errors > 0,
this change will restart the resilver immediately instead of doing
the rest of the scan and then restarting it from the beginning. On
the other hand, if scn_errors == 0 at import, then no errors have
been encountered so far, so the spa_scrub_started flag can be safely
set.

A test has been added to verify that resilver does not restart when
relevant DTL's are available.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Paul Zuchowski <[email protected]>
Signed-off-by: John Poduska <[email protected]>
Closes #10291 

2 files changed, 43 insertions, 2 deletions

diff --git a/module/zfs/dsl_scan.c b/module/zfs/dsl_scan.c
index 74ef2e155..f09501793 100644
--- a/module/zfs/dsl_scan.c
+++ b/module/zfs/dsl_scan.c
@@ -542,6 +542,22 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 			zfs_dbgmsg("new-style scrub was modified "
 			    "by old software; restarting in txg %llu",
 			    (longlong_t)scn->scn_restart_txg);
+		} else if (dsl_scan_resilvering(dp)) {
+			/*
+			 * If a resilver is in progress and there are already
+			 * errors, restart it instead of finishing this scan and
+			 * then restarting it. If there haven't been any errors
+			 * then remember that the incore DTL is valid.
+			 */
+			if (scn->scn_phys.scn_errors > 0) {
+				scn->scn_restart_txg = txg;
+				zfs_dbgmsg("resilver can't excise DTL_MISSING "
+				    "when finished; restarting in txg %llu",
+				    (u_longlong_t)scn->scn_restart_txg);
+			} else {
+				/* it's safe to excise DTL when finished */
+				spa->spa_scrub_started = B_TRUE;
+			}
 		}
 	}
 
@@ -887,7 +903,6 @@ dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
 		    "errors=%llu", (u_longlong_t)spa_get_errlog_size(spa));
 
 	if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
-		spa->spa_scrub_started = B_FALSE;
 		spa->spa_scrub_active = B_FALSE;
 
 		/*
@@ -915,6 +930,12 @@ dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
 		spa_errlog_rotate(spa);
 
 		/*
+		 * Don't clear flag until after vdev_dtl_reassess to ensure that
+		 * DTL_MISSING will get updated when possible.
+		 */
+		spa->spa_scrub_started = B_FALSE;
+
+		/*
 		 * We may have finished replacing a device.
 		 * Let the async thread assess this and handle the detach.
 		 */
diff --git a/module/zfs/vdev.c b/module/zfs/vdev.c
index 3c2135029..923bf2e33 100644
--- a/module/zfs/vdev.c
+++ b/module/zfs/vdev.c
@@ -2583,7 +2583,6 @@ vdev_dtl_should_excise(vdev_t *vd)
 	spa_t *spa = vd->vdev_spa;
 	dsl_scan_t *scn = spa->spa_dsl_pool->dp_scan;
 
-	ASSERT0(scn->scn_phys.scn_errors);
 	ASSERT0(vd->vdev_children);
 
 	if (vd->vdev_state < VDEV_STATE_DEGRADED)
@@ -2634,6 +2633,7 @@ vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg, int scrub_done)
 
 	if (vd->vdev_ops->vdev_op_leaf) {
 		dsl_scan_t *scn = spa->spa_dsl_pool->dp_scan;
+		boolean_t wasempty = B_TRUE;
 
 		mutex_enter(&vd->vdev_dtl_lock);
 
@@ -2643,6 +2643,18 @@ vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg, int scrub_done)
 		if (zfs_scan_ignore_errors && scn)
 			scn->scn_phys.scn_errors = 0;
 
+		if (scrub_txg != 0 &&
+		    !range_tree_is_empty(vd->vdev_dtl[DTL_MISSING])) {
+			wasempty = B_FALSE;
+			zfs_dbgmsg("guid:%llu txg:%llu scrub:%llu started:%d "
+			    "dtl:%llu/%llu errors:%llu",
+			    (u_longlong_t)vd->vdev_guid, (u_longlong_t)txg,
+			    (u_longlong_t)scrub_txg, spa->spa_scrub_started,
+			    (u_longlong_t)vdev_dtl_min(vd),
+			    (u_longlong_t)vdev_dtl_max(vd),
+			    (u_longlong_t)(scn ? scn->scn_phys.scn_errors : 0));
+		}
+
 		/*
 		 * If we've completed a scan cleanly then determine
 		 * if this vdev should remove any DTLs. We only want to
@@ -2679,6 +2691,14 @@ vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg, int scrub_done)
 			space_reftree_generate_map(&reftree,
 			    vd->vdev_dtl[DTL_MISSING], 1);
 			space_reftree_destroy(&reftree);
+
+			if (!range_tree_is_empty(vd->vdev_dtl[DTL_MISSING])) {
+				zfs_dbgmsg("update DTL_MISSING:%llu/%llu",
+				    (u_longlong_t)vdev_dtl_min(vd),
+				    (u_longlong_t)vdev_dtl_max(vd));
+			} else if (!wasempty) {
+				zfs_dbgmsg("DTL_MISSING is now empty");
+			}
 		}
 		range_tree_vacate(vd->vdev_dtl[DTL_PARTIAL], NULL, NULL);
 		range_tree_walk(vd->vdev_dtl[DTL_MISSING],