[libvirt-users] [Qemu-devel] About VM fork in QEMU

[Xinyang Ge]

>>> External snapshots (via the blockdev-snapshot-sync QMP command) can be
>>> taken in a matter of milliseconds if you only care about disk state.
>>> Furthermore, if you want to take a snapshot of both memory and disk
>>> state, such that the clone can be resumed from the same time, you can do
>>> that with a guest downtime that only lasts as long as the
>>> blockdev-snapshot-sync, by first doing a migrate to file then doing the
>>> disk snapshot when the VM pauses at the end of migration.  Resuming the
>>> original guest is fast; resuming from the migration file is a bit
>>> longer, but it is still the fastest way possible to resume from a
>>> memory+disk snapshot.  If you need anything faster, then yes, you would
>>> have to write patches to qemu to attempt cloning via fork() that makes
>>> sure to modify the active disk in use by the fork child so as not to
>>> interfere with the fork parent.
>>
>> I think migrating memory to file then doing external disk snapshot is
>> exactly what we want. Since we are using libvirt to manage different
>> VMs, could you give us some specific guides (or references) that how
>> we could migrate memory state to file using virsh interfaces and do
>> external snapshots?
>
> virsh snapshot-create-as $dom $name --live --memspec /path/to/memoryfile

I have tried this command on libvirt v1.1.3 and it returns "error:
invalid argument: qemuDomainSnapshotCreateXML: unsupported flags
(0x100)". Looks like --live is not supported yet. Could you let us
know which version we should of libvirt we should use in order to use
this feature?

Thanks,
Xinyang

-- 
Xinyang GE
Department of Computer Science & Engineering
The Pennsylvania State University
Homepage: http://www.cse.psu.edu/~xxg113/