md/raid6: Fix anomily when recovering a single device in RAID6.

NeilBrown · shligit · commit 7471fb77ce4d · 2017-04-10T10:35:27.000-07:00
When recoverying a single missing/failed device in a RAID6, those stripes where the Q block is on the missing device are handled a bit differently. In these cases it is easy to check that the P block is correct, so we do. This results in the P block be destroy. Consequently the P block needs to be read a second time in order to compute Q. This causes lots of seeks and hurts performance. It shouldn't be necessary to re-read P as it can be computed from the DATA. But we only compute blocks on missing devices, since c337869 ("md: do not compute parity unless it is on a failed drive"). So relax the change made in that commit to allow computing of the P block in a RAID6 which it is the only missing that block. This makes RAID6 recovery run much faster as the disk just "before" the recovering device is no longer seeking back-and-forth. Reported-by-tested-by: Brad Campbell <lists2009@fnarfbargle.com> Reviewed-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: NeilBrown <neilb@suse.com> Signed-off-by: Shaohua Li <shli@fb.com>
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
@@ -3619,9 +3619,20 @@ static int fetch_block(struct stripe_head *sh, struct stripe_head_state *s,
 		BUG_ON(test_bit(R5_Wantcompute, &dev->flags));
 		BUG_ON(test_bit(R5_Wantread, &dev->flags));
 		BUG_ON(sh->batch_head);
+
+		/*
+		 * In the raid6 case if the only non-uptodate disk is P
+		 * then we already trusted P to compute the other failed
+		 * drives. It is safe to compute rather than re-read P.
+		 * In other cases we only compute blocks from failed
+		 * devices, otherwise check/repair might fail to detect
+		 * a real inconsistency.
+		 */
+
 		if ((s->uptodate == disks - 1) &&
+		    ((sh->qd_idx >= 0 && sh->pd_idx == disk_idx) ||
 		    (s->failed && (disk_idx == s->failed_num[0] ||
-				   disk_idx == s->failed_num[1]))) {
+				   disk_idx == s->failed_num[1])))) {
 			/* have disk failed, and we're requested to fetch it;
 			 * do compute it
 			 */
