[libvirt] How to avoid failure of migration/restoring/starting if cdrom is ejected inside guest?

[Eric Blake]

On 08/08/2011 09:22 AM, Dave Allan wrote:
>> If the medium is ejected inside guest, and the source of the medium
>> is removed or renamed. libvirt will still tries to do cgroup setting
>> before starting the guest on dest side, that's the real problem. As
>> one will think it's reasonable to remove the source if it's
>> ejected. And we can't prevent one remove the source during the
>> migration (supposing the removing is before libvirt tries to do
>> cgroup setting on dest host.)
>
> Agreed, but that's a problem that I don't think we can solve except by
> documenting the behavior.  The libvirt documentation has always
> clearly stated that the storage configuration must be the same on the
> src and dst hosts.  We are discussing altering that constraint here in
> the case of medium that has been ejected, but I think we can be clear
> that the constraint is not being removed and that a guest's storage
> configuration must not be changed while a migration is in progress.

I thought the qemu folks were working on a series of patches to make cd 
tray state part of the migration information.  That is, if you start a 
migration, then a guest ejects the cd, then the destination will 
correctly see the ejected cd, even though the destination was started 
with the tray closed.  That is, libvirt should not have to poll on cd 
tray state in order to correctly migrate, but should be able to poll on 
cd tray state in order to report tray state to the curious user.

The migration aspect of cd tray state generally has to be solved by qemu 
- there's nothing libvirt can do if the guest changes cd tray state 
after migration has started but before it has completed, unless libvirt 
can poll the source after it has paused but before the destination has 
been unpaused and issue appropriate monitor commands on the destination 
to resync state; but that should only be necessary if qemu doesn't 
migrate cd tray state in the first place, and requires that qemu support 
tray state monitor commands for libvirt to use.

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