Re: [Qemu-devel] KVM call agenda for June 28

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jul 05, 2011 at 01:40:08PM +0100, Stefan Hajnoczi wrote:
> On Tue, Jul 5, 2011 at 9:01 AM, Dor Laor <dlaor@redhat.com> wrote:
> > I tried to re-arrange all of the requirements and use cases using this wiki
> > page: http://wiki.qemu.org/Features/LiveBlockMigration
> >
> > It would be the best to agree upon the most interesting use cases (while we
> > make sure we cover future ones) and agree to them.
> > The next step is to set the interface for all the various verbs since the
> > implementation seems to be converging.
> 
> Live block copy was supposed to support snapshot merge.  I think the
> current favored approach is to make the source image a backing file to
> the destination image and essentially do image streaming.
> 
> Using this mechanism for snapshot merge is tricky.  The COW file
> already uses the read-only snapshot base image.  So now we cannot
> trivally copy the COW file contents back into the snapshot base image
> using live block copy.

It never did. Live copy creates a new image were both snapshot and
"current" are copied to.

This is similar with image streaming.

> It seems like snapshot merge will require dedicated code that reads
> the allocated clusters from the COW file and writes them back into the
> base image.
> 
> A very inefficient alternative would be to create a third image, the
> "merge" image file, which has the COW file as its backing file:
> snapshot (base) -> cow -> merge
> 
> All data from snapshot and cow is copied into merge and then snapshot
> and cow can be deleted.  But this approach is results in full data
> copying and uses potentially 3x space if cow is close to the size of
> snapshot.

Management can set a higher limit on the size of data that is merged,
and create a new base once exceeded. This avoids copying excessive
amounts of data.

> Any other ideas that reuse live block copy for snapshot merge?
> 
> Stefan