[Libguestfs] Inspection of disk snapshots

[Kashyap Chamarthy]

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