[Libvir] Snapshots for Xen and KVM

[Daniel Veillard]

On Tue, Mar 11, 2008 at 04:13:40PM +0000, John Levon wrote:
> On Tue, Mar 11, 2008 at 03:43:51PM +0000, Richard W.M. Jones wrote:
> 
> > On Tue, Mar 11, 2008 at 12:54:37PM +0200, Gabriel Kaufmann wrote:
> > > Does anyone knows if Xen and KVM support snapshots (regardless if
> > > Libvirt provides the API for them).
> > 
> > It's currently on the Xen to-do list, but apart from some research
> > papers I'm not aware of them implementing it.  Perhaps recently.
> > Checkpointing is tricky to get right because you have to take a
> > snapshot of the memory and disk at exactly the same moment.
> > 
> > Note that neither Xen's live migration nor the domain save features
> > are real checkpoints, because they both only consider the memory.
> 
> Why would Xen itself be at all concerned with snapshotting the disk?
> AIUI, there's hooks in xend so you just plug in your disk snapshotting
> (ZFS, LVM, vmdk, whatever) to that.

  I think we discussed that on the list a little bit before, basically
the current state both for KVM and for Xen is that if you have a very
specific environment, using special filesystems, special hooks, then you
may be able to do a working checkpointing. The big problem I see is that
from an API point of view there is no way to make that a predictable
reliable interface in the current state of the support of the hypervisors.
I feel a bit uneasy doing this, maybe I'm wrong we already have APIs
where there is an uncertainty about the result of the operation 
(virDomainShutdown) but in that case it's likely that an undetected
failed operation means data corruption, and we just can't detect properly
(nor does the hypervisor e.g. iSCSI file systems) that this will fail.
So I don't feel the state of support at the hypervisor level allows to
export this in good faith at the libvirt level, but maybe I'm wrong and
I would accept to be ruled out of course :-)

Daniel

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