linux-ext4 - Re: MD/DM and barriers (was Re: [patch] ext2/3: document conditions when reliable operation is possible)

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Date:	Tue, 1 Sep 2009 16:01:21 +0200
From:	Pavel Machek <pavel@....cz>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Ric Wheeler <rwheeler@...hat.com>, Theodore Tso <tytso@....edu>,
	Rob Landley <rob@...dley.net>, Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: Re: MD/DM and barriers (was Re: [patch] ext2/3: document
	conditions when reliable operation is possible)

On Thu 2009-08-27 12:54:05, Jeff Garzik wrote:
> On 08/27/2009 09:10 AM, Ric Wheeler wrote:
>> One thing that does need fixing for some MD configurations is to stress
>> again that we need to make sure that barrier operations are properly
>> supported or users will need to disable the write cache on devices with
>> volatile write caches.
>
> Agreed; chime in on Christoph's linux-vfs thread if people have input.
>
> I quickly glanced at MD and DM.  Currently, upstream, we see a lot of
>
>         if (unlikely(bio_barrier(bio))) {
>                 bio_endio(bio, -EOPNOTSUPP);
>                 return 0;
>         }
>
> in DM and MD make_request functions.
>
> Only md/raid1 supports barriers at present, it seems.  None of the other  
> MD drivers support barriers.

Not even md/raid0? Ouch :-(.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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