linux-kernel - Re: Barriers still not passing on simple dm devices...

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Date:	Mon, 23 Mar 2009 22:52:31 +0100
From:	Bodo Eggert <7eggert@....de>
To:	Eric Sandeen <sandeen@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Andi Kleen <andi@...stfloor.org>,
	 ?missing closing '"' in token?"MASON",
	CHRISTOPHER <CHRIS.MASON@...cle.com>
Subject: Re: Barriers still not passing on simple dm devices...

Eric Sandeen <sandeen@...hat.com> wrote:

> I've noticed that on 2.6.29-rcX, with Andi's patch
> (ab4c1424882be9cd70b89abf2b484add355712fa, dm: support barriers on
> simple devices) barriers are still getting rejected on these simple devices.

> So what's the right way around this?  What should dm (or md for that
> matter) advertise on their queues about ordered-ness?  Should there be
> some sort of "QUEUE_ORDERED_PASSTHROUGH" or something to say "this level
> doesn't care, ask the next level" or somesuch?  Or should it inherit the
> flag from the next level down?  Ideas?

IMO each block device SHOULD provide barriers - either native or emulated,
and each meta device MUST provide emulated barriers if it provides dynamic
device binding and barriers.

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