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

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Message-ID: <20081205115225.13c277ee@lxorguk.ukuu.org.uk>
Date:	Fri, 5 Dec 2008 11:52:25 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	xfs@....sgi.com, Alasdair G Kergon <agk@...hat.com>,
	Andi Kleen <andi-suse@...stfloor.org>,
	Milan Broz <mbroz@...hat.com>
Subject: Re: Device loses barrier support (was: Fixed patch for simple
 barriers.)

On Fri, 5 Dec 2008 02:37:39 +0100
Andi Kleen <andi@...stfloor.org> 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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