[Libguestfs] Inspection of disk snapshots

[Richard W.M. Jones]

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.

> 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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