On Mon, Mar 23, 2015 at 10:43:30PM +0000, Richard W.M. Jones wrote: [. . .]
This makes a copy of the whole disk image. It's also not a consistent (point in time) copy.
Oh I see that you're copying the _snapshot_ that you created with libvirt; it's not a whole disk copy. There's still not any point in doing this, and what I said below stands.
At that point I mount it through libguestfs and inspect its content.
As long as you use the 'readonly=1' flag (which is really *essential*, else you'll get disk corruption), you can just point libguestfs at the base image:
g = guestfs.GuestFS (python_return_dict=True) g.add_drive_opts ("base_image.qcow2", format="qcow2", readonly=1)
That also doesn't get you a consistent snapshot, but it'll work most of the time, and give you a clear error in libguestfs when it doesn't (and won't corrupt your base disk or anything like that, provided you're using readonly=1).
The effect of the readonly=1 flag is to create an external snapshot. It is roughly the equivalent of doing:
qemu-img create -f qcow2 -b base_image snapshot.qcow2 < point libguestfs at snapshot.qcow2 >
If you want lightweight, consistent, point-in-time snapshots (which it sounds like you do), qemu has recently been adding this capability. See the 'drive-backup' monitor command. I've not tried using that
A small QMP test for 'drive-backup': https://kashyapc.fedorapeople.org/virt/test-qmp-drive-backup.txt
and I don't know if it is wired up through libvirt,
I don't see it wired up yet in libvirt current `git` master.
but libguestfs should be able to consume it since it's just an NBD source.
-- /kashyap