[libvirt-users] Mounting VM filesystem on host while VM running

[Daniel P. Berrange]

On Mon, Aug 08, 2016 at 10:19:29AM +0200, Michal Privoznik wrote:
> On 08.08.2016 09:33, Patrick PICHON wrote:
> > Hello,
> > 
> > 
> > All my VMs are using LVs created on the host side.
> > 
> > I'm using collectd to monitor some of the ressources of my host as well
> > as the libvirt pluging to monitor my VMs.
> > 
> > Collectd has an interesting plugin ( df ) which can monitor the
> > filesystem usage.
> > 
> > I would like to use it to monitor the VMs filesytsem usage.
> > 
> > As it is obvious that if I mount the VM filesystem on the host while the
> > VM is running, I'm wondering if I could mount it in READONLY without
> > expecting any panic/corruption problem.
> > 
> > 
> > Any advices ?
> 
> I don't think it is a good idea. Mostly because of caching - you
> probably won't get consistent results. Moreover, some filesystems (e.g.
> ext4) replay journal even if mounted as RO (frankly, I haven't found a
> way how to mount ext really RO). I don't know of any filesystem that
> would support being mounted multiple times from two different hosts
> (except network FS obviously).

Yep, mounting the guest FS on the host is practically guaranteed
to lead to data loss. Don't even think about trying it.

> What you could do is to write small guest agent that will report disk
> usage to host (e.g. via virito serial console).

FWIW, you can run the libguestfs guest agent inside a running guest and
then connect to it using normal libguestfs API

Regards,
Daniel
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