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. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top