[dm-devel] dm-thin: Export proper discard_granularity

[Mike Snitzer]

On Wed, Jun 11 2014 at  8:32am -0400,
Lukas Czerner <lczerner at redhat.com> wrote:

> Currently if the underlying device is discard capable and the
> discard_passdown is enabled, the discard_granularity will be inherited
> from that device.
> 
> This will pose a problem in the case that the device discard_granularity
> is smaller than thin volume chunk size, because in that case discard
> requests will not be chunk size aligned so it will be ignored by
> dm-thin.
> 
> Fix this by setting thin volume discard granularity to the bigger of the
> two max(device discard_granularity, thin volume chunk size). Strictly
> speaking it is not necessary to get the bigger of the two, because
> the thin volume chunk size will always be >= device discard_granularity.
> However I believe that the reason for this is only because dm-thin can
> not handle discard requests bigger than chunk size which is hopefully
> going to change soon. This way it is future proof.
> 
> RHBZ: 1106856
> 
> Reported-by: Zdenek Kabelac <zkabelac at fedoraproject.org>
> Signed-off-by: Lukas Czerner <lczerner at redhat.com>

Hi Lukas,

I missed this submission on dm-devel until now.  But I had already
picked this patch up earlier from the BZ, see the patch I staged in
linux-next here:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=09869de57ed2728ae3c619803932a86cb0e2c4f8