[Xen-users] [LVM2 + DRBD + Xen + DRBD 8.0] errors on dom0 and on domU

[Maxim Doucet <maxim@xxxxxxxxxxxx>]

This message was originally posted in the DRBD mailing list
(http://lists.linbit.com/pipermail/drbd-user/2007-August/007261.html).
I decided to post it here too to reach a broader audience.
I'll do my best to report any interesting to each mailing list if it can
diserve this issue.
The issue seems to involve both Xen and DRBD ; trying to centralize some
information between them doesn't sound too silly for me...

Here is the issue :

I'll try with this message to link together different discussions which
are dealing with this setup : LVM2 + DRBD + Xen + DRBD 8.0

I have the following environment :

 * 2 identical servers with Fedora Core 7 and it's XEN 3.1 (kernel-xen
2.6.20-2925.13.fc7xen)

 * DRBD 8.0.5 primary/primary working (without LinuxHA for now)

 * LVM on top of DRBD, with LVM correctly configured to reject the disk
under the DRBD device : /dev/drbd0 with VGvmstor0 on top

 * I have only one paravirtualized Virtual Machine (VM) which is using a
file (not a partition nor a LVM's Logical Volume) as it's disk storage.
The VM's storage file is on the root partition of dom0 (-> the physical
server, the file being located in /var/lib/xen/images), so : not on LVM
and thus not on the DRBD device. The VM just runs fine.

Now, while the virtual machine is off, I change its configuration to add
a new device :
 * the source storage is a LVM's Logical Volume located on dom0. It is
located on the DRBD resource : /dev/VGvmstor0/LVvmsys1
 * the newly created device which is seen by the virtual machine (the
domU) is : /dev/xdvb

I experience the following error messages when launching the virtual
machine :
*On dom0 : the physical server* (messages coming from dmesg) :
drbd0: bio would need to, but cannot, be split:
(vcnt=2,idx=0,size=2048,sector=126353855)
drbd0: bio would need to, but cannot, be split:
(vcnt=2,idx=0,size=2048,sector=126353855)
drbd0: bio would need to, but cannot, be split:
(vcnt=8,idx=0,size=4096,sector=126562682)
drbd0: bio would need to, but cannot, be split:
(vcnt=8,idx=0,size=4096,sector=126562750)
drbd0: bio would need to, but cannot, be split:
(vcnt=4,idx=0,size=4096,sector=126562495)
drbd0: bio would need to, but cannot, be split:
(vcnt=4,idx=0,size=4096,sector=126562495)
drbd0: bio would need to, but cannot, be split:
(vcnt=4,idx=0,size=4096,sector=126353855)
drbd0: bio would need to, but cannot, be split:
(vcnt=4,idx=0,size=4096,sector=126353855)
... and further


*On domU : the virtual machine* (messages appearing at boot time, on the
console) :
blkfront: xvdb: barriers enabled
 xvdb: xvdb1 xvdb2
end_request: I/O error, dev xvdb, sector 208703
Buffer I/O error on device xvdb1, logical block 104320
Buffer I/O error on device xvdb1, logical block 104321
Buffer I/O error on device xvdb1, logical block 104322
Buffer I/O error on device xvdb1, logical block 104323
end_request: I/O error, dev xvdb, sector 208703
Buffer I/O error on device xvdb1, logical block 104320
Buffer I/O error on device xvdb1, logical block 104321
Buffer I/O error on device xvdb1, logical block 104322
Buffer I/O error on device xvdb1, logical block 104323
end_request: I/O error, dev xvdb, sector 10473981
Buffer I/O error on device xvdb2, logical block 10265136
Buffer I/O error on device xvdb2, logical block 10265137
... and further


Now, after searching for information about these messages through the
web, I've found the following threads which seems to be linked to the
problem :
 1. http://forums.xensource.com/message.jspa?messageID=4666#4666 :
The beginning of the thread is not about the error "bio would need to,
but cannot, be split" but the post I link (number #4666) is reporting it
and points to :

 2. ...the following patch :
http://lists.linbit.com/pipermail/drbd-user/2007-April/006677.html

 3. http://lists.linbit.com/pipermail/drbd-user/2007-June/007054.html :
reports the same error "bio would need to, but cannot, be split" in the
same configuration


Question : according to
http://lists.linbit.com/pipermail/drbd-user/2007-April/006728.html : is
this problem linked with the errors on dom0 ("bio would need to, but
cannot, be split") ? If yes, was it fixed (I'll forward this mail to the
xen-user mailing list).
By now, is there any workaround
(http://forums.xensource.com/message.jspa?messageID=4666#4699 talks
about a modified dm-mod.ko? or maybe the patch in
http://lists.linbit.com/pipermail/drbd-user/2007-April/006677.html) ?

Post http://forums.xensource.com/message.jspa?messageID=4666#4669
reports that everything works well with DRBD 7. Does it means that, for
now, DRBD 8.0 can't be used on top of a dom0 Xen Kernel ?
-- 
Maxim Doucet - www.alamaison.fr
sys admin @ la maison




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