[dm-devel] Ext4 and xfs problems in dm-thin on allocation and discard

[Lukáš Czerner]

On Tue, 19 Jun 2012, Spelic wrote:

> Date: Tue, 19 Jun 2012 13:29:55 +0200
> From: Spelic <spelic at shiftmail.org>
> To: Lukáš Czerner <lczerner at redhat.com>
> Cc: Mike Snitzer <snitzer at redhat.com>, Dave Chinner <david at fromorbit.com>,
>     Spelic <spelic at shiftmail.org>,
>     device-mapper development <dm-devel at redhat.com>,
>     linux-ext4 at vger.kernel.org, xfs at oss.sgi.com
> Subject: Re: Ext4 and xfs problems in dm-thin on allocation and discard
> 
> On 06/19/12 08:32, Lukáš Czerner wrote:
> > 
> > So do I understand correctly that even though the discard came
> > through and thinp took advantage of it it still returns EOPNOTSUPP ?
> > This seems rather suboptimal. IIRC there was a discussion to add an
> > option to enable/disable sending discard in thinp target down
> > to the device.
> 
> I'll ask this too...
> do I understand correctly that dm-thin returns EOPNOTSUPP to the filesystem
> layer even though it is using the discard to unmap blocks, and at that point
> XFS stops sending discards down there (while ext4 keeps sending them)?
> 
> This looks like a bug of dm-thin to me. Discards are "supported" in such a
> scenario.
> 
> Do you have a patch for dm-thin so to prevent it sending EOPTNOTSUPP ?

Yes, definitely this behaviour need to change in dm-thin. I do not
have a path, it was merely a proposal how thing could be done. Not
sure what Mike and rest of the dm folks think about this.

-Lukas

> 
> Thank you
> S.
>