Re: [Qemu-devel] [Bug 1184089] Re: [Feature request] loadvm snapshot as read-only

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, May 27, 2013 at 10:42:17PM -0000, Michael Coppola wrote:
> Awesome, looking forward to it.  I may be misunderstanding what's
> happening under the hood, but at least for me, calling 'loadvm' on a
> single snapshot over and over seems to work the first few times and then
> immediately blue screens the WinXP guest with PFN_LIST_CORRUPT.  I was
> under the assumption that all runtime modifications were being written
> back to the image, effectively "corrupting" something (whether it was
> changes to the snapshot or the "backing image" causing things to break).

savevm/loadvm does not use backing images.  It relies on internal
snapshot which are stored inside the existing qcow2 image file.

If you *are* using backing images then you're right - modifying the
backing image is likely to trigger weird guest behavior.

> Until then, I've seemed to have found a workaround for the feature
> itself.  Instead of creating a snapshot with 'savevm', I can start the
> VM with -snapshot and then call:
> 
> migrate "exec: gzip -c > snapshot.gz"
> 
> in QMP and it saves the live image to a compressed file.  Make sure it's
> completed migration before exiting with "info migrate".  Subsequently
> loading the snapshot with:
> 
> qemu-* <whatever flags> -incoming "exec: gzip -c -d snapshot.gz"
> -snapshot
> 
> will load the live snapshot and redirect all runtime modifications to a
> temp file.  http://www.linux-kvm.org/page/Migration says not to use
> -snapshot, but who follows the rules anyways? ;)  It seems to work so
> far and things haven't exploded yet.  Running md5sum on the qcow2 image
> and gzip snapshot before and after shows no changes to either files.

The reason that -snapshot isn't used together with migration is because
the disk state will be discarded and not migrated.

Stefan