Re: md: raid5 resync corrects read errors on data block - is this correct?

[NeilBrown <neilb@suse.de>]

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On Mon, 17 Sep 2012 14:15:16 +0300 Alexander Lyakas <alex.bolshoy@gmail.com>
wrote:

> Hi Neil,
> below is a bit less-ugly version of the patch.
> Thanks,
> Alex.
> 
> >From 05cf800d623bf558c99d542cf8bf083c85b7e5d5 Mon Sep 17 00:00:00 2001
> From: Alex Lyakas <alex@zadarastorage.com>
> Date: Thu, 13 Sep 2012 18:55:00 +0300
> Subject: [PATCH] When RAID5 is dirty, force reconstruct-write instead of
>  read-modify-write.
> 
> Signed-off-by: Alex Lyakas <alex@zadarastorage.com>
> Signed-off-by: Yair Hershko <yair@zadarastorage.com>
> 
> diff --git a/ubuntu_kmodules/Ubuntu-3.2.0-25.40/drivers/md/raid5.c
> b/ubuntu_kmodules/Ubuntu-3.2.0-25.40/drivers/md/raid5.c
> index 5332202..0702785 100644
> --- a/ubuntu_kmodules/Ubuntu-3.2.0-25.40/drivers/md/raid5.c
> +++ b/ubuntu_kmodules/Ubuntu-3.2.0-25.40/drivers/md/raid5.c
> @@ -2555,12 +2555,36 @@ static void handle_stripe_dirtying(struct r5conf *conf,
>                                    int disks)
>  {
>         int rmw = 0, rcw = 0, i;
> -       if (conf->max_degraded == 2) {
> -               /* RAID6 requires 'rcw' in current implementation
> -                * Calculate the real rcw later - for now fake it
> +       sector_t recovery_cp = conf->mddev->recovery_cp;
> +       unsigned long recovery = conf->mddev->recovery;
> +       int needed = test_bit(MD_RECOVERY_NEEDED, &recovery);
> +       int resyncing = test_bit(MD_RECOVERY_SYNC, &recovery) &&
> +                       !test_bit(MD_RECOVERY_REQUESTED, &recovery) &&
> +                       !test_bit(MD_RECOVERY_CHECK, &recovery);
> +       int transitional = test_bit(MD_RECOVERY_RUNNING, &recovery) &&
> +                          !test_bit(MD_RECOVERY_SYNC, &recovery) &&
> +                          !test_bit(MD_RECOVERY_RECOVER, &recovery) &&
> +                          !test_bit(MD_RECOVERY_DONE, &recovery) &&
> +                          !test_bit(MD_RECOVERY_RESHAPE, &recovery);

Thanks Alex,
 however I don't understand why you want to test all of these bits.
Isn't it enough just to check ->recovery_cp ??

> +
> +       /* RAID6 requires 'rcw' in current implementation.
> +        * Otherwise, attempt to check whether resync is now happening
> +        * or should start.
> +         * If yes, then the array is dirty (after unclean shutdown or
> +         * initial creation), so parity in some stripes might be inconsistent.
> +         * In this case, we need to always do reconstruct-write, to ensure
> +         * that in case of drive failure or read-error correction, we
> +         * generate correct data from the parity.
> +         */
> +       if (conf->max_degraded == 2 ||
> +           (recovery_cp < MaxSector && sh->sector >= recovery_cp &&
> +            (needed || resyncing || transitional))) {
> +               /* Calculate the real rcw later - for now fake it
>                  * look like rcw is cheaper

Also, we should probably fix this comment.  s/fake/make/

Thanks,
NeilBrown



>                  */
>                 rcw = 1; rmw = 2;
> +               pr_debug("force RCW max_degraded=%u, recovery_cp=%lu
> sh->sector=%lu recovery=0x%lx\n",
> +                        conf->max_degraded, recovery_cp, sh->sector, recovery);
>         } else for (i = disks; i--; ) {
>                 /* would I have to read this buffer for read_modify_write */
>                 struct r5dev *dev = &sh->dev[i];


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