[DRBD-user] DRBD Freezing

Lars Ellenberg Lars.Ellenberg at linbit.com
Mon Feb 5 20:08:18 CET 2007

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

/ 2007-02-05 13:05:46 -0500
\ Brian E. Dunbar:
> I am having a problem with DRBD occasionally freezing on me. Before I give
> to much information let me hit one topic that may be the root cause.
> The most relevant piece of setup information may be that the disk space that
> drbd is using for storage is located on a loopback file system created with
> losetup. I mention this because I have since found some posts on this list
> and other drbd web pages that this setup may in fact be the problem (but
> there did not seem to be a conclusive answer), even though some the patches
> in the 0.7.X tree seem to address this very situation. Can anyone tell me
> whether using the loopback file systems is indeed a problem?
> If you don't think that is the problem here is some more information. We had
> this freezing problem very frequently when we first installed drbd with a
> 0.6.X version (what came on Ubuntu). I found references to fixes in the
> 0.7.X tree for deadlocking and upgraded and that seemed to solve the problem
> (ran for about 3 months without seeing it again). Until this weekend, when
> DRBD froze again.

drbd on top of a loop device is expected to deadlock sooner or later
due to resource starvation in the block io path. 

you'd hit this sooner with drbd 0.6, or with drbd 0.7 and kernel 2.4.
you can probably still hit this with the most recent drbd and kernel.

if you don't have much io load, and you do have much free memory, you
probably could even run this for a long time and never have problems.
but eventually it will deadlock.


it _may_ work (though I'm not too sure about that),
if the loop device is block-device-backed.

it _will_ deadlock if the loop device is file-backed,
due to block io re-entering the file-io path.

: Lars Ellenberg                            Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH      Fax +43-1-8178292-82 :
: Vivenotgasse 48, A-1120 Vienna/Europe    http://www.linbit.com :
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