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

[Richard W.M. Jones]

On Wed, Mar 25, 2015 at 07:38:03PM +0100, Kashyap Chamarthy wrote:
> 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

That's interesting, but even more interesting would be to see
drive-backup writing to NBD.

>From what I gather, drive-backup would take a nbd:... target, meaning
that it is the *client* (not the server).  What's interesting is what
you're supposed to connect to the server.  (Maybe I'm wrong about this
however)

Rich.

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