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

From: Andi Kleen <andi@firstfloor.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Andi Kleen <andi-suse@firstfloor.org>,
	linux-kernel@vger.kernel.org, xfs@oss.sgi.com,
	Andi Kleen <andi@firstfloor.org>,
	Alasdair G Kergon <agk@redhat.com>, Milan Broz <mbroz@redhat.com>
Subject: Re: Device loses barrier support (was: Fixed patch for simple barriers.)
Date: Fri, 5 Dec 2008 02:37:39 +0100	[thread overview]
Message-ID: <20081205013739.GZ6703@one.firstfloor.org> (raw)
In-Reply-To: <Pine.LNX.4.64.0812041948400.14114@hs20-bc2-1.build.redhat.com>

> * 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@linux.intel.com

_______________________________________________
xfs mailing list
xfs@oss.sgi.com
http://oss.sgi.com/mailman/listinfo/xfs