[libvirt] How to avoid the disk still persistent after no persistent hotplug and with restarting libvirtd then.

[Eric Blake]

On 08/19/2011 08:17 AM, Osier Yang wrote:
> Libvirt loads the domain conf from the status file
> (/var/run/libvirt/qemu/$dom.xml)
> if the domain is running, the problem if one restart libvirtd just after
> some changing on the domain conf, and then destroy/start the guest.
> libvirt will never known what the original domain conf is then.
> (vm->newdef is NULL at this moment, generally libvirt restore the domain
> conf
> to the original conf (vm->newDef) when destroying and shutdown).

Sounds to me like this is a bug in libvirtd reloading.  For a running 
persistent guest, libvirt should be looking in two files: both the 
running state (/var/run/libvirt/qemu/$dom.xml) and the next-boot state 
(/etc/lib/libvirt/qemu/$dom.xml); and if they differ, then assign 
vm->newDef to the persistent definition.  In other words, the bug is 
that vm->newdef was NULL after libvirtd restart.

> The solution in my mind is to add some attribute to the status XML, such
> as "live_updated=1", and do the checking when loading the domain conf
> from status XML:

Shouldn't need any XML changes; rather, libvirtd just needs to be taught 
to properly reconstruct its state from multiple files, instead of 
assuming that the first file that worked was all the state it needed.

> For a transient domain, we still have no way to get the original domain
> conf,
> that means one will still be surprised when seeing the the disk is still
> there.

For a transient domain, it is already forbidden to request a persistent 
change.  So you can't get into this situation in the first place unless 
you have a persistent domain.

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