Re: raid1 repair: sync_request() aborts if one of the drives has bad block recorded

[NeilBrown <neilb@xxxxxxx>]

On Thu, 12 Jul 2012 18:38:43 +0300 Alexander Lyakas <alex.bolshoy@xxxxxxxxx>
wrote:

> Hi Neil,
> I am testing the following simple scenario:
> - RAID1 with two drives
> - First drive has a bad block marked in the bad block list
> - "repair" is triggered

Thanks for testing!!


> 
> What is happening is that the code in raid1.c:sync_request() selects
> candidates for reading. If it encounters a bad block recorded, it
> skips this particular drive:
> 			if (is_badblock(rdev, sector_nr, good_sectors,
> 					&first_bad, &bad_sectors)) {
> 				if (first_bad > sector_nr)
> 					.../* we don't go here*/
> 				else {
>                                         /* we go here*/
> 					bad_sectors -= (sector_nr - first_bad);
> 					if (min_bad == 0 ||
> 					    min_bad > bad_sectors)
> 						min_bad = bad_sectors;
> 				}
> 			}
> 			if (sector_nr < first_bad) {
>                             /* we don't go here */
>                             ...
> But the second drive has no bad blocks recorded, so it is selected.
> So we end up with read_targets=1 and min_bad>0.
> 
> Then the following code:
> 	if (test_bit(MD_RECOVERY_SYNC, &mddev->recovery) && read_targets > 0)
> 		/* extra read targets are also write targets */
> 		write_targets += read_targets-1;
> leaves write_targets=0
> 
> Then the following code:
> 	if (write_targets == 0 || read_targets == 0) {
> 		/* There is nowhere to write, so all non-sync
> 		 * drives must be failed - so we are finished
> 		 */
> 		sector_t rv = max_sector - sector_nr;
> aborts the resync.

Oops.

> 
> Is this the intended behavior? Because it looks like this bit does not
> take into account the bad-blocks existence.
> I am not sure about which logic would solve it, but something like "If
> we did not select a drive for reading because of bad blocks, and as a
> result we have only one read_target, and MD_RECOVERY_REQUESTED, then
> let's report only the bad block as skipped and not the whole drive".
> Something like that, but there are probably many cases I am not
> thinking about.
> 
> What do you think?

I don't see that MD_RECOVERY_REQUESTED is really an issue is it?
We the wrong thing happen in any case where there just one device with no bad
block, and at least one device with a bad block.  In that case we want to
skip forward by the number of bad blocks, not skip to the end of the array.

So this?

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 240ff31..c443f93 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -2422,7 +2422,10 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipp
 		/* There is nowhere to write, so all non-sync
 		 * drives must be failed - so we are finished
 		 */
-		sector_t rv = max_sector - sector_nr;
+		sector_t rv;
+		if (min_bad > 0)
+			max_sector = sector_nr + min_bad;
+		rv = max_sector - sector_nr;
 		*skipped = 1;
 		put_buf(r1_bio);
 		return rv;


Thanks,
NeilBrown

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