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

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Message-ID: <4A96BA2D.60804@garzik.org>
Date:	Thu, 27 Aug 2009 12:54:05 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	Theodore Tso <tytso@....edu>, Rob Landley <rob@...dley.net>,
	Pavel Machek <pavel@....cz>, 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: MD/DM and barriers (was Re: [patch] ext2/3: document conditions when
 reliable operation is possible)

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.

DM has some barrier code...  but the above code was pasted from DM's 
make_request function, so I am guessing that DM's barrier stuff is 
incomplete and disabled at present.

I've been mentioning this issue for years... glad some people finally 
noticed :)

	Jeff



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