Well, if you had a closer look at the BZs that you've quoted, especially BZ 1219541, you should have seen that this problem apparently has been solved in upstream QEMU version 2.9 or 2.10. So it's really just about the backlevel version 2.5 that you're using. I'm re-assigning this ticket to the Ubuntu package, maybe they can fix it for you.
-- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1449687 Title: block migration of qcow2 VMs copies all empty space Status in qemu package in Ubuntu: New Bug description: I'm running openstack 2012.1 'icehouse' which, ultimately, calls down into qemu-system-x86 2.0.0+dfsg-2ubuntu1.10. I primed the process by copying all necessary base images onto the destination host. Nonetheless, post-migration instances are much larger than the original image; the copy duplicated all the empty space that ought to have remained copy-on-write. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1449687/+subscriptions
