6:32 a.m.
On Tue, Mar 24, 2015 at 10:54:05AM +0200, NoxDaFox wrote:
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?
Well we have a tool called virt-diff
(http://libguestfs.org/virt-diff.1.html) which prints the differences
between two disks. It's quite commonly used to show the differences
between an original base image and a snapshot taken some time later,
so you can tell which files have been modified by the guest.
Now virt-diff works by opening both disks, reading all of the metadata
(or even the file content if you use the --checksum option), and then
internally diffing it and presenting the result.
Of course this means it's not especially fast, but it's the way that
it has to work: The snapshot doesn't contain "files which changed", it
contains underlying device blocks which changed. It operates a whole
layer or two below the filesystem.
To do this from Python is not particularly hard, but you'll have to
read the C and translate it. The guts of the algorithm are in the
recursive "visitor" mini-library:
https://github.com/libguestfs/libguestfs/blob/master/diff/diff.c
https://github.com/libguestfs/libguestfs/blob/master/cat/visit.h
https://github.com/libguestfs/libguestfs/blob/master/cat/visit.c
There are alternatives -- perhaps parsing the qcow2 snapshot, and
mapping disk blocks back to files -- but they won't be very easy to
implement. I wrote a highly experimental* tool called 'virt-bmap' that
may be of interest:
https://rwmj.wordpress.com/2014/11/23/mapping-files-to-disk/
https://rwmj.wordpress.com/2014/11/24/mapping-files-to-disk-part-2/
Rich.
* = if it breaks, you get to keep all the pieces
--
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