[Libguestfs] Inspection of disk snapshots

[Richard W.M. Jones]

On Mon, Mar 23, 2015 at 10:41:01PM +0000, Richard W.M. Jones wrote:
> On Mon, Mar 23, 2015 at 04:34:21PM +0200, NoxDaFox wrote:
> > Greetings,
> > 
> > I have the following typical scenario: given one or more qcow2 base images
> > I clone them with COW and start the VMs.
> > 
> > At a certain point I'd like to inspect them in order to see their evolution
> > compared to the known base images. To do so I was thinking about taking a
> > disk snapshot of each VM and inspect its content through libguestfs (using
> > it's Python bindings).
> >
> > Obviously I need the base image in order for libguestfs to correctly guess
> > the OS, the FS structure etc.. Problem is that that point when I inspect
> > the disk I get the whole disk state including the base image content (30K+
> > files and directories).
> > 
> > This is not an issue but it's a very heavy operation considering that some
> > of the snapshots are few megabytes while the base images are several
> > gigabytes.
> > 
> > Is there a way to programmatically instruct libguestfs to limit the
> > inspection to the sole snapshot?
> > Would it work as well with other disk format (vmdk linked clones for
> > example)?
> > 
> > For better comprehension I'll show the sequence I'm doing (I might do it
> > wrong of course):
> > 
> > virsh -c "qemu:///system" snapshot-create --disk-only <domain-ID>
> > 
> > I get the snapshot location from its XML description and then:
> > 
> > qemu-img convert -f qcow2 -O qcow2 base_image.qcow2 snapshot.qcow2
> 
> 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 and
> I don't know if it is wired up through libvirt, but libguestfs should
> be able to consume it since it's just an NBD source.

Rich.

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