[Xen-users] Concurrent local write error in DRBD with GPL PV drivers

[Philippe Lang <philippe.lang@xxxxxxxxxxxxxx>]

Hi,

We are about to start using in production Windows 2003 64bits XEN VMs running 
on top of a DRBD cluster. We have installed the latest GPL PV optimized 
drivers. (gplpv_2003x64_0.11.0.213.msi)

Sometimes, when the VM writes on disk, we can see the following error in the 
DRBD log:

May  6 17:45:48 s3 kernel: [198887.626841] drbd0: blkback.35.hda[3549] 
Concurrent local write detected! [DISCARD L] new: 26745079s +4096; pending: 
26745079s +4096

This error is discussed in the following thread of the DRBD mailing list:

http://lists.linbit.com/pipermail/drbd-user/2009-April/011873.html

Basically, it is due to the fact that a write occurs at a specific location, 
while another "in-flight" write is taking place at the same location. In order 
to avoid a cluster desynchronization, DRBD drops the second write.

We were able to reproduce this problem, with the help of a Windows program 
called "PerformanceTest" from Passmark Software. When doing a "Disk Random Seek 
+RW" test, the logs gets filled with the error mentioned at the top of this 
message.

We have tested a VM *without* the gplpv drivers, and *no error* appears. We 
have tested previous drivers versions (0.11.0.188, 0.10.0.142), and the same 
error appears.

So, we have the feeling there is some kind of error in the driver, although we 
have never experienced a single VM crash. Can we safely ignore the "Concurrent 
local write error" mentioned in the log, or is that really a bug that should be 
corrected before using the driver in production?

Best regards,

-------------------------------------------------------------
Attik System              web  : http://www.attiksystem.ch
Philippe Lang             phone: +41 26 422 13 75
rte de la Fonderie 2      gsm  : +41 79 351 49 94
1700 Fribourg             pgp  : http://keyserver.pgp.com

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