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

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.

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>

1 files changed, 12 insertions, 1 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index a5676559e7a6..09d94ad5e52b 100644
--- 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
                          */