Re: [Libvir] save/restore support for KVM
Daniel Veillard wrote:
On Thu, Aug 09, 2007 at 10:55:10PM +0100, Daniel P. Berrange wrote:
> Just been committed to KVM repos I see. Should be an easy patch to backport
> too. As long as we can detect failure if this is missing & report it back
> then I'm fine depending on this.
Would checking for the kvm version from the console sufficient ? Since KVM
makes even more releases than libvirt in average I guess that would be
fine.
I'm not sure the kvm qemu binary even reports the kvm version
anywhere. I'll ask on kvm-devel to see if qemu/VERSION could get
updated with each KVM release.
> > - I append the domain's UUID at the end of the
migration image.
> > This doesn't affect KVM at all (it ignores the extra data).
> > Does that seem reasonable? It's unclear how the saved image
> > is supposed to get associated with a particular VM configuration
> > without doing something like this.
>
> Actually I'd store the entire XML config appended to the end of the image.
> Its quite possible the saved image may be restored on a different machine
> so libvirt will need the XML config there & its not much work to automatically
> append it all & use it when restoring later.
+1 . The only problem is that the XML has no predefined size, so it may be
hard to stack more stuff behind it. I would ask first on the KVM list to check
if it's okay to add a variable lenght data structure at the end, they might
want to extend it in the future and that would be hard to handle.
I think appending unrelated data to the migration image is a bit of a
hack anyway. A better plan would be a file containing
<header> <XML config> <migration data>
On save, libvirt writes <header> and <XML config>, then closes it and
uses "dd of=path oflag=append conv=notrunc" or just "cat >>
path" as
the migration command.
On restore, libvirt reads the header and XML config, and then
feeds the remaining migration data to KVM using "-incoming stdio".
I had wanted to avoid the trouble of feeding data via stdin, but
maybe a well placed dup2(fd,STDIN_FILENO) would do the trick
automatically.
This file format would also make it easier for e.g. virt-manager to
determine that a file is a valid libvirt restore image.
-jim