[Libguestfs] Fwd: Inspection of disk snapshots

Tue Mar 24 08:54:05 UTC 2015

I was sure I was doing something wrong as I'm not yet fully aware of QCOW2
snapshot feature and how it interacts with libguestfs.

I'll try to explain better the scenario:

I have several hosts running lots of VMs which are generated from few base
images, say A, B, C the base images (backing file) and A1, A2, A*, B1, B2,
B* clones on top of which the newly spawned VMs are running.
I need to collect the disk states of A*, B*, C* machines and see what has
been written there. I don't care about the whole content as the base images
content A, B, C are well known to me, only thing it matters are the deltas
of the new clones.

One more piece in the puzzle is that the inspection does not happen on the
hosts running the VMs but on a dedicated server.

My idea was to collect those "snapshots" (generic term not the QEMU one)
from the hosts and send them to my inspection server. As A, B and C are
accessible from that server only thing I need is to rebase those snapshot
to correctly inspect them through libguestfs, and it proved to work (I'm
using readonly mode as I only care about reading the disks). I'm not really
interested in having consistent point-in-time state of the disks as the
operation is done several times a day so I can cope with semi-consistent
data as it can be easily re-constructed.

My real problem comes when I try to inspect the disk snapshot: libguestfs
will, of course, let me see the whole content of the disks, which means A +
A*. Apart from the waste of CPU time spend on looking at files I already
know the state (the ones contained in A), it generates a lot of noise. A
Linux base image with some library installed consists in 20+ K files,
installing something extra (Apache server for example) just brings some
hundreds new files and I'm interested only in those ones.

So my real question is: is there a way to distinguish the files contained
in the two different disk images (A and A1) or shall I think about a
totally different approach?

Thank you.

2015-03-24 0:43 GMT+02:00 Richard W.M. Jones <rjones at redhat.com>:

> 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
>
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