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Date: Wed, 25 May 2011 09:23:02 -0400
From: Mike Snitzer <snitzer@redhat.com>
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Subject: Re: [linux-lvm] Trim support on stripped volumes ?
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On Wed, May 25 2011 at  4:11am -0400,
Franck Routier (personnel) <alci@mecadu.org> wrote:

> Le mardi 24 mai 2011 � 09:14 -0400, Mike Snitzer a �crit :
> 
> > When using -o discard, ext4 will log a warning if a discard failed (and
> > will disable discards).
> > 
> > So ext4 is a great indicator for whether discards are working on your
> > striped LV volume.
> > 
> 
> Just out of curiosity, how does ext4 tell if discards is working or
> not ?

It checks the return code from ext4_issue_discard() -- which comes from
sb_issue_discard().  See: fs/ext4/mballoc.c:release_blocks_on_commit()