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

[xen@xxxxxxxxxx]

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



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