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@xxxxxxxxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
drivers/md/raid5.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index c523fd69a7bc..aeb2e236a247 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3617,9 +3617,20 @@ 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
*/
--
2.12.0
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