Re: [Libguestfs] Inspection of disk snapshots
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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