libvirt users - Jan 2013 - Sanlock gives up lock when VM is paused

Hello,

I'm using libvirt and sanlock on qemu-kvm guests. Each guest has it's 
own Logical Volume for it's root filesystem. Sanlock is configured and 
working and prevents me from starting the same VM twice on multiple 
nodes and corrupting it's root filesystem. Each VM's domain XML resides 
on 2 servers that share the LVM volume group over fiber channel.

In testing, I noticed that if I pause a VM on node 1, the sanlock lock 
is relinquished, and I am able to start the same VM, using the same root 
filesystem, on node 2. I get a lock error when unpausing node 1's VM if 
node 2's copy is still running, but by this point, the disk may already 
be corrupted.

Is it necessary that paused VMs don't get to keep their locks? Is there 
a way to configure sanlock to lock when a VM is paused?

Versions:
sanlock(-devel, -lib)    2.3-1.el6
libvirt(-lock-sanlock, -client)     0.9.10-21.el6_3.8

I'm using NFS for the lockspace.

Thanks,
Michael

-- 
Michael Rodrigues
Interim Help Desk Manager
Gevirtz Graduate School of Education
Education Building 4203
(805) 893-8031
help at education.ucsb.edu

On Thu, Jan 31, 2013 at 09:34:47AM -0800, Michael Rodrigues
wrote:
> Hello,
> 
> I'm using libvirt and sanlock on qemu-kvm guests. Each guest has
> it's own Logical Volume for it's root filesystem. Sanlock is
> configured and working and prevents me from starting the same VM
> twice on multiple nodes and corrupting it's root filesystem. Each
> VM's domain XML resides on 2 servers that share the LVM volume group
> over fiber channel.
> 
> In testing, I noticed that if I pause a VM on node 1, the sanlock
> lock is relinquished, and I am able to start the same VM, using the
> same root filesystem, on node 2. I get a lock error when unpausing
> node 1's VM if node 2's copy is still running, but by this point,
> the disk may already be corrupted.
The disk isn't corrupted - preventing node 1 VM from un-pausing
is explicitly what is preventing the disk from becoming corrupted.

Even if you now pause the VM on node 2, and try to resume node 1,
sanlock will still prevent node 1 from running again. It tracks
a lock "version" number. The key is that once the original VM
is paused, if any other VM runs on that disk, the orignal VM is
never allowed to be unpaused again.
> Is it necessary that paused VMs don't get to keep their locks? Is
> there a way to configure sanlock to lock when a VM is paused?
No, it isn't something that's configurable - this behaviour is
required in order todo migration for example.

Regards,
Daniel
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