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On Fri, Nov 02, 2012 at 03:54:02PM -0700, Vishvananda Ishaya wrote:
> So we dealt with this exact issue when doing snapshots for instances. According
> to the libvirt folks, creating and/or deleting a snapshot using qemu-image while
> a disk is in use is not safe.

It's indeed not.  Creating snapshots of any image format while qemu is
running requires doing it using the qemu monitor.  Note that this is the
exact same snapshot code that qemu-img would invoke, but doing it from
qemu means it is serialized vs I/O from the guest.

> Unfortunately libvirt does not expose a
> reasonable method for snapshotting and extracting data from a live disk,
> so the only way to do this safely is to use qemu-monitor (or patch libvirt to
> expose the right interface). This is why the instance snapshot command in kvm
> is doing a managed save before snapshotting the vm. If we had a good way to do
> this it would greatly help the instance snapshot code as well.

What do you mean with a managed save?