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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] KVM call agenda for June 28
Date: Tue, 5 Jul 2011 13:40:08 +0100

On Tue, Jul 5, 2011 at 9:01 AM, Dor Laor <address@hidden> 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 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

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


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