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Re: [libvirt] RFCv2: virDomainSnapshotCreateXML enhancements



Hello Kevin,

Am Freitag 12 August 2011 10:04:07 schrieb Kevin Wolf:
> Am 12.08.2011 09:18, schrieb Philipp Hahn:
> > On Thursday 11 August 2011 12:00:46 Kevin Wolf wrote:
> >> Am 11.08.2011 00:08, schrieb Eric Blake:
> >>> Libvirt currently has a bug in that it only saves <domain>/<uuid>
> >>> rather than the full domain xml along with a checkpoint - if any
> >>> devices are hot-plugged (or in the case of offline snapshots, if the
> >>> domain configuration is changed) after a snapshot but before the
> >>> revert, then things will most likely blow up due to the differences in
> >>> devices in use by qemu vs. the devices expected by the snapshot.
> >>
> >> Offline snapshot means that it's only a disk snapshot, so I don't think
> >> there is any problem with changing the hardware configuration before
> >> restoring it.
> >>
> >> Or does libvirt try to provide something like offline checkpoints, where
> >> restoring would not only restore the disk but also roll back the libvirt
> >> configuration?
> >
> > Try to load a VM state with the memory size changed in between and your
> > VM is busted; been there, experienced that :-( So it's nice to do a
> > snapshot before you play with your virtual hardware configuration and can
> > go back there if things go wrong.
> >
> > For -loadvm to work you have to call kvm with the nearly same command
> > line arguments again; what may change you probably know better than me.
> > For I thinks it's essential to store the VM consiguration with the
> > snapshot, which would be the qemu command line arguments, which is
> > equivalent to libvirts XML description.
>
> Yes, I understand this. I was talking about snapshots taken while the VM
> is shut off, where it's not as clear.

I think is very useful for offline snapshots too, since your VM might depends 
on the exact qemu command line: think of (incomplete) RAIDs or udevs 
persistent MAC address rules.

> But for consistency it's probably 
> better to do the same with offline and online snapshots, so what libvirt
> implements (or was it only Eric's plan?) is fine here.

Currently as of libvirt-0.9.4 the domain configuration is not saved with the 
snapshot. I have implemented that for our internal 0.8.4 version as a working 
proof-on-concept, but Eric is now reviewing the whole picture.

Sincerely
Philipp
-- 
Philipp Hahn           Open Source Software Engineer      hahn univention de
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/

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