linux-kernel - Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

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Message-ID: <20070529220500.GA6513@agk.fab.redhat.com>
Date:	Tue, 29 May 2007 23:05:00 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Stefan Bader <sbader3@...glemail.com>
Cc:	device-mapper development <dm-devel@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	David Chinner <dgc@....com>, Phillip Susi <psusi@....rr.com>,
	Stefan Bader <Stefan.Bader@...ibm.com>,
	Andreas Dilger <adilger@...sterfs.com>,
	Tejun Heo <htejun@...il.com>
Subject: Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

On Tue, May 29, 2007 at 11:25:42AM +0200, Stefan Bader wrote:
> doing a sort of suspend, issuing the
> barrier request, calling flush to all mapped devices and then wait for
> in-flight I/O to go to zero? 

Something like that is needed for some dm targets to support barriers.
(We needn't always wait for *all* in-flight I/O.)
When faced with -EOPNOTSUP, do all callers fall back to a sync in
the places a barrier would have been used, or are there any more
sophisticated strategies attempting to optimise code without barriers?

> I am not a hundred percent sure about
> that but I think that just passing the barrier flag on to mapped
> devices might in some (maybe they are rare) cases cause a layer above
> to think all data is on-disk while this isn't necessarily true (see my
> previous post). What do you think?

An efficient I/O barrier implementation would not normally involve
flushing AFAIK: dm surely wouldn't "cause" a higher layer to assume
stronger semantics than are provided.
 
Alasdair
-- 
agk@...hat.com
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