Re: Weird Issue with raid 5+0

[chris <tknchris@gmail.com>]

Interesting, I moved to raid6 on the machine I was working on because
it was similar enough and I had a deadline to meet. I would still be
interested in testing this though. With your approval I would like to
copy xen-devel on this thread so that we can hopefully put together a
fix. Thanks again for your help previously and for what looks to be
the solution :)

- chris

On Mon, Mar 8, 2010 at 12:50 AM, Neil Brown <neilb@suse.de> wrote:
> On Sun, 21 Feb 2010 19:16:40 +1100
> Neil Brown <neilb@suse.de> wrote:
>
>> On Sun, 21 Feb 2010 02:26:42 -0500
>> chris <tknchris@gmail.com> wrote:
>>
>> > That is exactly what I didn't want to hear :( I am running
>> > 2.6.26-2-xen-amd64. Are you sure its a kernel problem and nothing to
>> > do with my chunk/block sizes? If this is a bug what versions are
>> > affected, I'll build a new domU kernel and see if I can get it working
>> > there.
>> >
>> > - chris
>>
>> I'm absolutely sure it is a kernel bug.
>
> And I think I now know what the bug is.
>
> A patch was recently posted to dm-devel which I think addresses exactly this
> problem.
>
> I reproduce it below.
>
> NeilBrown
>
> -------------------
> If the lower device exposes a merge_bvec_fn,
> dm_set_device_limits() restricts max_sectors
> to PAGE_SIZE "just to be safe".
>
> This is not sufficient, however.
>
> If someone uses bio_add_page() to add 8 disjunct 512 byte partial
> pages to a bio, it would succeed, but could still cross a border
> of whatever restrictions are below us (e.g. raid10 stripe boundary).
> An attempted bio_split() would not succeed, because bi_vcnt is 8.
>
> One example that triggered this frequently is the xen io layer.
>
> raid10_make_request bug: can't convert block across chunks or bigger than 64k 209265151 1
>
> Signed-off-by: Lars <lars.ellenberg@linbit.com>
>
>
> ---
>  drivers/md/dm-table.c |   12 ++++++++++--
>  1 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 4b22feb..c686ff4 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -515,14 +515,22 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
>
>        /*
>         * Check if merge fn is supported.
> -        * If not we'll force DM to use PAGE_SIZE or
> +        * If not we'll force DM to use single bio_vec of PAGE_SIZE or
>         * smaller I/O, just to be safe.
>         */
>
> -       if (q->merge_bvec_fn && !ti->type->merge)
> +       if (q->merge_bvec_fn && !ti->type->merge) {
>                limits->max_sectors =
>                        min_not_zero(limits->max_sectors,
>                                     (unsigned int) (PAGE_SIZE >> 9));
> +               /* Restricting max_sectors is not enough.
> +                * If someone uses bio_add_page to add 8 disjunct 512 byte
> +                * partial pages to a bio, it would succeed,
> +                * but could still cross a border of whatever restrictions
> +                * are below us (e.g. raid0 stripe boundary).  An attempted
> +                * bio_split() would not succeed, because bi_vcnt is 8. */
> +               limits->max_segments = 1;
> +       }
>        return 0;
>  }
>  EXPORT_SYMBOL_GPL(dm_set_device_limits);
> --
> 1.6.3.3
>
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