linux-ext4 - Re: [Bug 14354] Status of barrier requests in LVM and dm-crypt?

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Message-ID: <20091013144945.GA22223@redhat.com>
Date:	Tue, 13 Oct 2009 10:49:45 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	dm-devel@...hat.com, linux-ext4@...r.kernel.org,
	bugzilla-daemon@...zilla.kernel.org,
	Mikulas Patocka <mpatocka@...hat.com>
Subject: Re: [Bug 14354] Status of barrier requests in LVM and dm-crypt?

On Mon, Oct 12 2009 at  2:15pm -0400,
Theodore Ts'o <tytso@....edu> wrote:

> Hi, I'm trying to track down a problem relating to ext4 problems, in
> kernel bugzilla 14354.  Originally it was thought this was a regression
> 2.6.31->2.6.32-rc1 regression, but the Aneesh Kumar's report shows a
> very similar fsck transcript using a 2.6.30-1 kernel (which makes makes
> it unclear whether this is really a regression or not), and all three
> reports are tied together by the use of device-mapper.  
> 
> The fsck logs are consistent barriers being disabled.  As I recall under
> some circumstances device mapper silently drops barrier requests --- I
> thought this was fixed for device mapper in some cases.  What's the
> current status of barrier requests in device mapper?  Are they
> faithfully passed for standard (non-RAID) LVM2 volumes?  When was this
> changed?   What about dm-crypto volumes?  (One of the reports was with a
> dm-crypt volume from what I can tell.)

Barrier infrastructure started to get added to the DM core in 2.6.30, see:
http://git.kernel.org/linus/af7e466a1ace

But barriers were not enabled for all DM targets (_except_ dm-multipath)
until 2.6.31.  So 2.6.31's dm-crypt does support barriers, see:
http://git.kernel.org/linus/647c7db14ef9

If the underlying device(s) support barriers DM should faithfully pass
them on (again except for dm-multipath).  Also, requests with barriers
that result in -EOPNOTSUPP are retried without the barrier, see:
http://git.kernel.org/linus/51aa32284958

Mike
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