[dm-crypt] Does dm-crypt support journaling filesystem transactional guarantees?

Arno Wagner arno at wagner.name
Tue Apr 24 10:14:09 CEST 2012

On Tue, Apr 24, 2012 at 09:47:42AM +0200, Milan Broz wrote:
> 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.)

See also FAQ item 2.10, which I just have updated to reflect this 
information ;-)


> > 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
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Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
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One of the painful things about our time is that those who feel certainty 
are stupid, and those with any imagination and understanding are filled 
with doubt and indecision. -- Bertrand Russell 

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