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

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Message-ID: <20081205013739.GZ6703@one.firstfloor.org>
Date:	Fri, 5 Dec 2008 02:37:39 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	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.)

> * 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.
> 
> * the filesystems developed hacks to work around this issue, the hacks 
> involve not submitting more requests after the barrier request, 

I suspect the reason the file systems did it this way is that
it was a much simpler change than to rewrite the transaction
manager for this.

> synchronously waiting for the barrier request and eventually retrying it. 
> These hacks suppress any performance advantage barriers could bring.
> 
> * you submit a patch that makes barriers even more often deviate from the 
> specification and you argue that the patch is correct because filesystems 
> handle this deviation.

Sorry what counts is the code behaviour, not the specification.

-Andi

-- 
ak@...ux.intel.com
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