[Libguestfs] Point-in-time snapshots (was: Re: Inspection of disk snapshots)

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

From what I gather, drive-backup would take a nbd:... target, meaning