[dm-devel] max_sectors_kb limitations with VDO and dm-thin

[Ryan Norwood]

Great, I will do that. Thank you for all your help!

On Wed, Apr 24, 2019 at 6:22 PM Mike Snitzer <snitzer at redhat.com> wrote:

> On Wed, Apr 24 2019 at  5:27pm -0400,
> Mike Snitzer <snitzer at redhat.com> wrote:
>
> >
> > On Wed, Apr 24 2019 at 10:46am -0400,
> > Ryan Norwood <ryan.p.norwood at gmail.com> wrote:
> >
> > >    On Wed, Apr 24, 2019 at 9:08 AM Ryan Norwood <[1]
> ryan.p.norwood at gmail.com>
> > >    wrote:
> > >
> > >      Thank you for your help.
> > >      You are correct, it appears that the problem occurs when there is
> a RAID
> > >      5 or RAID 50 volume beneath VDO.
> > >      NAME      KNAME    RA   SIZE ALIGNMENT  MIN-IO  OPT-IO PHY-SEC
> LOG-SEC
> > >      RQ-SIZE SCHED    WSAME
> > >      sdh
> > >       sdh     128 977.5G         0     512       0     512     512
>  128
> > >      deadline    0B
> > >      +-sed6
> > >      dm-6    128 977.5G         0     512       0     512     512
>  128
> > >               0B
> > >        +-md127
> > >       md127 12288   5.7T         0 1048576 6291456     512     512
>  128
> > >               0B
> > >          +-vdo_data
> > >      dm-17   128   5.7T         0 1048576 6291456     512     512
>  128
> > >               0B
> > >            +-vdo
> > >       dm-18   128  57.3T         0    4096    4096    4096    4096
>  128
> > >               0B
>
> <snip>
>
> > >      /sys/block/dm-18/queue/max_hw_sectors_kb:4
> > >      /sys/block/dm-18/queue/max_sectors_kb:4
>
> These are getting set as a side-effect of MD raid imposing the need for
> merge_bvec (in the context of RHEL7.x only, not upstream) otherwise it
> goes conservative and forces the IO to be contrained to a single page,
> please see:
>
> drivers/md/dm-table.c:dm_set_device_limits() at the end:
>
>         /*
>          * Check if merge fn is supported.
>          * If not we'll force DM to use PAGE_SIZE or
>          * smaller I/O, just to be safe.
>          */
>         if (dm_queue_merge_is_compulsory(q) && !ti->type->merge)
>                 blk_limits_max_hw_sectors(limits,
>                                           (unsigned int) (PAGE_SIZE >> 9));
>
> With MD raid in the IO stack, dm_queue_merge_is_compulsory() will return
> true, so the VDO target not providing ti->type->merge causes this issue.
>
> Please file a BZ at bugzilla.redhat.com against VDO and I'll continue to
> work with the VDO developers to get this fixed for you for RHEL7.5, etc.
>
> Thanks,
> Mike
>
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