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



fyi - end of a very long thread, but I thought that dm had this fixed...

ric


-------- Original Message --------
Subject: MD/DM and barriers (was Re: [patch] ext2/3: document conditions when reliable operation is possible)
Date: 	Thu, 27 Aug 2009 12:54:05 -0400
From: 	Jeff Garzik <jeff garzik org>
To: 	Ric Wheeler <rwheeler redhat com>
CC: Theodore Tso <tytso mit edu>, Rob Landley <rob landley net>, Pavel Machek <pavel ucw cz>, Florian Weimer <fweimer bfk de>, Goswin von Brederlow <goswin-v-b web de>, kernel list <linux-kernel vger kernel org>, Andrew Morton <akpm osdl org>, mtk manpages gmail com, rdunlap xenotime net, linux-doc vger kernel org, linux-ext4 vger kernel org, corbet lwn net



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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