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Date: Tue, 24 Apr 2012 09:47:42 +0200
From: Milan Broz <mbroz@redhat.com>
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Subject: Re: [dm-crypt] Does dm-crypt support journaling filesystem
 transactional guarantees?
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To: Yang Zhang <yanghatespam@gmail.com>
Cc: dm-crypt@saout.de

On 04/24/2012 08:45 AM, Yang Zhang wrote:
> I'm considering using ext4 on encrypted LVM (which uses LUKS and
> dm-crypt). Will the transactional guarantees in ext4's journaling be
> preserved?

yes. dm-crypt operates in block layer, so it is filesystem responsibility
to properly set needed bits for IO (flush cache, FUA - force unit access)
and dmcrypt (device-mapper in general and block layer) must process them.

(read http://lwn.net/Articles/400541/ for more info).

In short, ext4, xfs, btrfs and similar filesystems supports
all features over dmcrypt.

(Except very old kernels, but most of stable distros backports patches.)
 
> Bonus: where may I find authoritative information on this (besides the source)?
The source is your best friend :-)

basically this is the first commit introducing real barrier
(later replaced with FUA) support
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=647c7db14ef9cacc4ccb3683e206b61f0de6dc2b

You can also use blktrace to see how are fs requests propagated
to physical media.

Milan