[libvirt] [openstack-dev] [nova] live-snapshot/cloning of virtual machines

Richard W.M. Jones rjones at redhat.com
Fri Aug 16 10:34:22 UTC 2013

On Fri, Aug 16, 2013 at 11:05:19AM +0100, Daniel P. Berrange wrote:
> On Wed, Aug 14, 2013 at 04:53:01PM -0700, Vishvananda Ishaya wrote:
> > Hi Everyone,
> > 
> > I have been trying for some time to get the code for the live-snapshot blueprint[1]
> > in. Going through the review process for the rpc and interface code[2] was easy. I
> > suspect the api-extension code[3] will also be relatively trivial to get in. The
> > main concern is with the libvirt driver implementation[4]. I'd like to discuss the
> > concerns and see if we can make some progress.
> > 
> > Short Summary (tl;dr)
> > =====================
> > 
> > I propose we merge live-cloning as an experimental feature for havanna and have the
> > api extension disabled by default.
> > 
> > Overview
> > ========
> >
> > First of all, let me express the value of live snapshoting. The
> > slowest part of the vm provisioning process is generally booting
> > of the OS.

Like Dan I'm dubious about this whole plan.  But this ^^ statement in
particular.  I would like to see hard data to back this up.

You should be able to boot an OS pretty quickly, and furthermore it's
(a) much safer for all the reasons Dan outlines, and (b) improvements
that you make to boot times help everyone.

> > 2. Security Concerns
> > ====================
> > 
> > There are a number of security issues with loading state from another vm. Here is a
> > short list of things that need to be done just to make a cloned vm usable:
> > 
> > a) mac address needs to be recreated
> > b) entropy pool needs to be reset
> > c) host name must be reset
> > d) host keys bust be regenerated
> > 
> > There are others, and trying to clone a running application as well may expose other
> > sensitive data, especially if users are snaphsoting vms and making them public.

Are we talking about cloning VMs that you already trust, or cloning
random VMs and allowing random other users to use them?  These would
lead to very different solutions.  In the first case, you only care
about correctness, not security.  In the second case, you care about
security as well as correctness.

I highly doubt the second case is possible because scrubbing the disk
is going to take far too long for any supposed time-saving to matter.

As Dan says, even the first case is dubious because it won't be correct.

> The libguestfs project provide tools to perform offline cloning of
> VM disk images.  Its virt-sysprep knows how to delete alot (but by
> no means all possible) sensitive file data for common Linux &
> Windows OS. It still has to be combined with use of the
> virt-sparsify tool though, to ensure the deleted data is actually
> purged from the VM disk image as well as the filesystem, by
> releasing all unused VM disk sectors back to the host storage (and
> not all storage supports that).

Links to the tools that Dan mentions:


Note these tools can only be used on offline machines.


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

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