From 7471fb77ce4dc4cb81291189947fcdf621a97987 Mon Sep 17 00:00:00 2001 From: NeilBrown Date: Mon, 3 Apr 2017 12:11:32 +1000 Subject: 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 Reviewed-by: Dan Williams Signed-off-by: NeilBrown Signed-off-by: Shaohua Li --- 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 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 */ -- cgit v1.2.3