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Re: Device loses barrier support (was: Fixed patch for simple barriers.)

To: Andi Kleen <andi@xxxxxxxxxxxxxx>
Subject: Re: Device loses barrier support (was: Fixed patch for simple barriers.)
From: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Date: Fri, 5 Dec 2008 11:52:25 +0000
Cc: Mikulas Patocka <mpatocka@xxxxxxxxxx>, Andi Kleen <andi@xxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, Alasdair G Kergon <agk@xxxxxxxxxx>, Andi Kleen <andi-suse@xxxxxxxxxxxxxx>, Milan Broz <mbroz@xxxxxxxxxx>
In-reply-to: <20081205013739.GZ6703@one.firstfloor.org>
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References: <20081204142015.GQ6703@one.firstfloor.org> <Pine.LNX.4.64.0812040913510.6118@hs20-bc2-1.build.redhat.com> <20081204145810.GR6703@one.firstfloor.org> <Pine.LNX.4.64.0812041139200.2434@hs20-bc2-1.build.redhat.com> <20081204174838.GS6703@one.firstfloor.org> <Pine.LNX.4.64.0812041401210.23079@hs20-bc2-1.build.redhat.com> <20081204221551.GV6703@one.firstfloor.org> <Pine.LNX.4.64.0812041756550.29638@hs20-bc2-1.build.redhat.com> <20081205004849.GX6703@one.firstfloor.org> <Pine.LNX.4.64.0812041948400.14114@hs20-bc2-1.build.redhat.com> <20081205013739.GZ6703@one.firstfloor.org>
On Fri, 5 Dec 2008 02:37:39 +0100
Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:

> > * barrier support in md-raid1 deviates from the specification at 
> > Documentation/block/barrier.txt. The specification says that requests 
> > submitted after the barrier request hit the media after the barrier 
> > request hits the media. The reality is that the barrier request can be 
> > randomly aborted and the requests submitted after it hit the media before 
> > the barrier request.
> 
> Yes the spec should be probably updated.
> 
> But also see Linus' rant from yesterday about code vs documentation.
> When in doubt the code wins.

Not when the fundamental design of the code is broken and trashes
performance. The documented behaviour is strongly desirable.

Alan

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