Patch "md/raid6: Fix anomily when recovering a single device in RAID6." has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
md-raid6-fix-anomily-when-recovering-a-single-device-in-raid6.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: NeilBrown <[email protected]>
Date: Mon, 3 Apr 2017 12:11:32 +1000
Subject: md/raid6: Fix anomily when recovering a single device in RAID6.

From: NeilBrown <[email protected]>

[ Upstream commit 7471fb77ce4dc4cb81291189947fcdf621a97987 ]

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 c337869d9501 ("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.

---
 drivers/md/raid5.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

@@ -3391,9 +3391,20 @@ static int fetch_block(struct stripe_hea
 		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
 			 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/md-raid6-fix-anomily-when-recovering-a-single-device-in-raid6.patch