[libvirt] snapshots += domain description?

[Eric Blake]

On 12/07/2010 09:00 AM, Philipp Hahn wrote:
> Hello,
> 
> I just encountered a problem with the current snapshot implementation for 
> qemu/kvm: To revert back to a snapshot, the domain description must closely 
> match the domain description when the snapshot was created. If the ram-size 
> doesn't match or the VM now contains fewer CPUs or contains additional disks, 
> kvm will either not load the snapshot or the machine will be broken 
> afterwarts.
> Is this a known problem?

Are you referring to a disk snapshot, created via 'virsh snapshot' on a
qcow2 image, or a memory snapshot, created via 'virsh save'?  The latter
_does_ save the guest XML as part of the snapshot, and 'virsh restore'
uses that saved XML rather than the current domain definition to resume
the guest.

> 
> Without looking into the implementation details I think saving the full domain 
> description instead of just a reference to the domain uuid as part of the 
> snapshot description would work better.

So it sounds like you are talking about the 'virsh snapshot' family of
commands.  Indeed, there's probably quite a bit of work to do in that
code to make it more reliable.

> Or am I supposed to save the domain 
> description myself in addition the the data saved 
> in /var/lib/libvirt/qemu/snapshot/$DOMAIN_NAME/$SNAPSHOT_NAME.xml. I think 
> this is very important feature, because when you - for example - notice that 
> you need additional disk space and would like to include an additional block 
> device, this is your ideal time to take a snapshot you can revert to when you 
> do something wrong.

I think we are hitting on more fundamental issues.  Right now, the
'virsh snapshot' family of commands is geared towards qcow2 images, and
relies heavily on the qcow2 internal snapshotting feature.  It would be
nice to enhance that to support other means of snapshotting (such as raw
disk images stored on an lvm partition or using btrfs file cloning), as
well as making live snapshots possible (by capturing memory as well as
disk images, or even coordinating with the guest to quiesce its file
systems so that restoring from a snapshot is less likely to restore a
file system in an inconsistent state).

> 
> Looking at VMware I see that reverting a VM there will not only revert the 
> disks and ram back to the saved state, but also the description including 
> name, ram size, etc.
> 
> Any comment or advise is appreciated.

Likewise - I'm hoping to look into this more myself, and any thoughts or
requirements that people can think of that should drive a decent
snapshot design should be brought up sooner rather than later.

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

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