From: Alasdair G Kergon <agk@redhat.com>
Subject: Re: [PATCH 4/4] Support discard if at least one underlying device
	supports it
Date: Fri, 2 Jul 2010 21:54:20 +0100
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Cc: Mikulas Patocka <mpatocka@redhat.com>, dm-devel@redhat.com,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	axboe@kernel.dk, dmonakhov@openvz.org
To: Mike Snitzer <snitzer@redhat.com>
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On Fri, Jul 02, 2010 at 04:47:09PM -0400, Mike Snitzer wrote:

> If a region doesn't support the discard
> DM has no business processing anything related to the discard (barriers
> included).  It is as simple as that.
 
Indeed - if an I/O is going to fail, discover that as early as we can, trying
to avoid the relatively-expensive barrier process whenever we reasonably can.

Alasdair