[libvirt-users] How do you force a VM reboot (when its KPd etc) without interrupting a blockcopy?

[Eric Blake]

On 01/16/2015 10:21 AM, Scott Sullivan wrote:
> My question is this:
> 
> If you have an ongoing blockcopy (drive-mirror) of a running transient VM, and the VM kernel panics, can you restart the VM without interrupting the ongoing blockcopy?

Sadly, this is not yet possible.  There is work being done in upstream
qemu to add persistent bitmap support, and the libvirt would have to be
taught to drive it.  The idea is that a persistent bitmap would allow
you to resume a copy operation from where it left off, rather than
having to start from scratch.  But as that is not implemented yet, the
best you can do is restart the copy from scratch.

You can minimize the penalty of a copy, though, by being careful about
what is being copied.  If you take an external snapshot prior to
starting a blockcopy, then you have a setup where the backing file is
large and stable, while the delta qcow2 wrapper is still relatively
small.  Copy the backing file via external means, if you have something
more efficient such as a storage pool cloner.  Meanwhile, use a shallow
blockcopy of just the qcow2 wrapper.  If that has to be restarted, it is
a much smaller file to redo; also, because it is smaller, a shallow
blockcopy is likely to finish faster.  Then, once everything is copied
to the new location, use active blockcommit to merge the temporary
wrapper back into the normal backing file.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 604 bytes
Desc: OpenPGP digital signature
URL: <http://listman.redhat.com/archives/libvirt-users/attachments/20150116/3aad8d6a/attachment.sig>