Date: Mon, 23 Jan 2006 17:01:47 +0000
From: Alasdair G Kergon <agk@redhat.com>
To: Andrew Morton <akpm@osdl.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/9] device-mapper snapshot: barriers not supported
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	Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
References: <20060120211759.GG4724@agk.surrey.redhat.com> <20060122214111.11170cdc.akpm@osdl.org>
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On Sun, Jan 22, 2006 at 09:41:11PM -0800, Andrew Morton wrote:
> Alasdair G Kergon <agk@redhat.com> wrote:
> > The snapshot and origin targets are incapable of handling barriers and 
> >  need to indicate this.

> And what was happening if people _were_ sending such BIOs down?  Did it all
> appear to work correctly?  

The snapshot target ignores the barrier and in some circumstances I/O can be
reordered in ways that are meant to be prevented by the barrier.

> If so, will this change cause
> currently-apparently-working setups to stop working?

As Lars pointed out, filesystems should already cope with -EOPNOTSUPP 
transparently, and it would be sensible for any out-of-tree users to 
do likewise.

Alasdair
-- 
agk@redhat.com
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