On Wed, May 10, 2017 at 03:16:20PM +0200, Tomasz Majchrzak wrote:
> If an array consists of two drives and the first drive has the bad
> block, the read request to the region overlapping the bad block chooses
> the same disk (with bad block) as device to read from over and over and
> the request gets stuck. If the first disk only partially overlaps with
> bad block, it becomes a candidate ("best disk") for shorter range of
> sectors. The second disk is capable of reading the entire requested
> range and it is updated accordingly, however it is not recorded as a
> best device for the request. In the end the request is sent to the first
> disk to read entire range of sectors. It fails and is re-tried in a
> moment but with the same outcome.
>
> Actually it is quite likely scenario but it had little exposure in my
> test until commit 715d40b93b10 ("md/raid1: add failfast handling for
> reads.") removed preference for idle disk. Such scenario had been passing
> as second disk was always chosen when idle.
>
> Update a candidate ("best disk") to read from if disk can read entire
> range. Do it only if other disk has already been chosen as a candidate
> for a smaller range. Otherwise, don't update it - let the head position /
> disk type logic to select a disk.
>
> Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@xxxxxxxxx>
> ---
> drivers/md/raid1.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> v2:
> updated the title not to include internal defect number :)
>
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index a1f3fbe..e7ab3bb 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -628,8 +628,11 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
> break;
> }
> continue;
> - } else
> + } else {
> + if ((sectors > best_good_sectors) && (best_disk >= 0))
> + best_disk = disk;
best_disk = -1 seems better here to me. If this is the only disk, we will
choose in below 'if (best_disk == -1)' check. If there are more disks which
cover the whole range, we will choose the best disk with minimum seek/pending.
Thanks,
Shaohua
> best_good_sectors = sectors;
> + }
>
> if (best_disk >= 0)
> /* At least two disks to choose from so failfast is OK */
> --
> 1.8.3.1
>
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