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

Yang Zhang yanghatespam at gmail.com
Tue Apr 24 10:16:45 CEST 2012


On Tue, Apr 24, 2012 at 1:14 AM, Arno Wagner <arno at wagner.name> wrote:
> 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 ;-)

At first I was confused what this was referring to - for posterity,
turns out this is the (very useful) cryptsetup FAQ at
http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions

Thanks!

>
> Arno
>
>
>
>> > 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
> GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
> ----
> 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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-- 
Yang Zhang
http://yz.mit.edu/


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