[DRBD-user] [LVM2 + DRBD + Xen + DRBD 8.0] error on dom0 (the physical server) and on domU (the virtual machine)

Maxim Doucet maxim at alamaison.fr
Thu Aug 16 10:35:25 CEST 2007


Sorry for the duplicate, a problem with my email client.

Here, is a forwarded email from the xen-users mailing list where someone
encountered the same problem.

A workaround is given, and further testing is done so I can only
recommend to read it.

The forwarded mail
(http://lists.xensource.com/archives/html/xen-users/2007-08/msg00375.html) 
:
> On Tue, 14 Aug 2007, Maxim Doucet wrote:
>
>> 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)
>
> We are using a nearly identical configuration and experienced the same
> problem just today:
>
> LVM2 on DRBD under Xen 3.0.3 w/ DRBD 8.0.4 Using CentOS5 on x86_64
> dom0 kernel 2.6.18-8.1.8-el5xen
>
> The virtual machine is an FC6 x86_64 PV guest and gave similar guest
> errors.
>
> The workaround we are using is to change
>
> disk = [ 'phy:/dev/vg-drbd/vm0,xvda,w' ]
>    to
> disk = [ 'tap:aio:/dev/vg-drbd/vm0,xvda,w' ]
>
> This treats the underlying backing image as a file.  This may have
> some performance loss since it is not using direct device IO, but as
> far as I can tell it is stable.  Or at least, phy: fails miserably,
> where tap:aio: works fine!
>
> This seems to indicate that its not an LVM+DRBD or Xen+LVM problem,
> but rather a Xen+LVM+DRBD using phy: problem.  I tested to see if Xen
> liked running LVM on a loopback device and loading a VM off it using
> phy: (see below).  It worked fine, which makes me think this is more
> of a drbd issue than a Xen or LVM issue.
>
> If you are on the DRBD list, please cross-post this (as I am not)
> since it is probably relevant.
>
> -Eric
>
>
> ============== Xen+LVM+loop test:
>
> # dd if=/dev/zero bs=1G seek=32 count=1 of=/tmp/testimage
> # losetup /dev/loop0 /tmp/testimage
> # pvcreate /dev/loop0
> # vgcreate vg-loop /dev/loop0
> # pvscan
>   [...]
>   PV /dev/loop0   VG vg-loop   lvm2 [11.00 GB / 6.99 GB free]
>   [...]
> # lvcreate -n testvm -l 1025 vg-loop
>
> # lvascan
>   [...]
>   ACTIVE            '/dev/vg-loop/testvm' [4.00 GB] inherit
>   [...]
>
> # ls -l
> -rwxr-xr-x 1 root root 4294967297 Jul 23 16:22 disk0
> # dd if=disk0 bs=4M of=/dev/vg-loop/testvm
> 1024+1 records in
> 1024+1 records out
> 4294967297 bytes (4.3 GB) copied, 396.227 seconds, 10.8 MB/s
>

-- 
Maxim Doucet - www.alamaison.fr
sys admin @ la maison




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