[dm-devel] Query about dm device max_discard_sectors

[Amar Mudrankit]

On Sun, Mar 25, 2012 at 9:49 PM, Mike Snitzer <snitzer at redhat.com> wrote:

> On Sun, Mar 25 2012 at  7:54pm -0400,
> Mike Snitzer <snitzer at redhat.com> wrote:
>
> > On Sun, Mar 25 2012 at  6:46pm -0400,
> > Amar Mudrankit <amar.mudrankit at gmail.com> wrote:
> >
> > > Hi all,
> > >
> > > I am building an experimental device mapper target which is supposed to
> > > receive
> > > all TRIM requests (REQ_DISCARD).  Irrespective of whether underlying
> actual
> > > physical block device supports discards or not, my device mapper target
> > > should
> > > receive REQ_DISCARD bios.
> >
> > I'm just curious but: what will your target do in general?
>

Target is somewhat similar to what a Flash Translation Layer does in SSDs,
but not
as fancy as that.  So, when my target maintains logical -> physical
mapping, my target
will be interested in which blocks are free from file system perspective, a
typical use
case of TRIM.  I guess somewhat similar to thin-provisioning.

>
> > > In my target, I set:
> > >
> > > ti->num_discard_requests = 1;
> > > ti->discards_supported = 1;
> >
> > You'll also want to set ti->discards_supported
> >
> > see: http://git.kernel.org/linus/4c259327
>
> ah, I now see you already did set discards_supported.
>
> > > So, if my device mapper target does not implement iterate_devices
> function
> > > (used to set max_discard_sectors based on underlying physical device's
> > > discard
> > > support) or underlying physical device does not support discards, then
> there
> > > is no way for device mapper target to set max_discard_sectors more
> than 0
> > > to receive REQ_DISCARD commands.
> > >
> > > Is this analysis correct? Or I am missing something?
> >
> > All DM targets should implement .iterate_devices -- intent is to iterate
> > over all data devices.
> >
> > You'll also want to implement .io_hints to set
> limits->max_discard_sectors
> > (like drivers/md/dm-thin.c does).
>
> BTW, the dm-thin.c discard changes will be merged in 3.4, you can see
> the relevant patches here:
>
>
> http://people.redhat.com/agk/patches/linux/editing/dm-thin-support-discards.patch
>
> http://people.redhat.com/agk/patches/linux/editing/dm_thin-add-pool-target-flags-to-control-discard.patch
>

With io_hints function pointer, I set max_discard_sectors and it is getting
set properly,  and I
was able to receive and process TRIM requests.

Thank you Mike for the help !!

Thanks and Regards,
Amar
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/dm-devel/attachments/20120326/a7682d56/attachment.htm>