[libvirt-users] Wanted: method for qemu hook script to know if called for migration

[Eric Blake]

On 07/25/2011 03:25 PM, Whit Blauvelt wrote:
> In trying to understand whether sanlock will work in this instance, I see at
> http://permalink.gmane.org/gmane.comp.emulators.libvirt/40261:
>
>    sanlock will ... protect against 2 vms on the same host using the same
>    disk (or the same VM being started twice due to error by libvirt).
>
> By implication if I have the sanlock directory on a shared filesystem, it
> will also prevent a VM from being started on the second host while running
> on the first. Will it still allow the VM to do a live migration? Or will
> live migration run up against the lock?

Yes, both useful lock managers (sanlock and fcntl) work best with the 
use of a shared lock directory, at which point they correctly handle 
live migration between hosts.  That is, the lock manager knows how to 
differentiate between read-only and read-write disk access, and the 
migration pattern is such that while the VM temporarily lives on two 
hosts at once, the migration API coordinates those two instances so that 
only one ever needs read-write access of a shared disk at a time.  The 
lock manager hooks are correctly called in the right sequence to account 
for migration, while still preventing you from starting up the same VM 
twice on multiple hosts via any means other than migration.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org