[libvirt-users] sharing read-only LOCAL disks among KVM guests

[Daniel P. Berrange]

On Fri, Sep 21, 2012 at 09:34:02PM +1000, Amos Shapira wrote:
> On 21 September 2012 21:30, Daniel P. Berrange <berrange at redhat.com> wrote:
> 
> > On Fri, Sep 21, 2012 at 09:22:24PM +1000, Amos Shapira wrote:
> > > Hello,
> > >
> > > Is it possible to share a single host's Logical Volume among multiple
> > local
> > > KVM guests which mount it read-only?
> > >
> > > I'm asking this because I have an idea to run multiple idential KVM
> > guests
> > > (they all have exactly the same software installed on them), booting them
> > > from a shared local Logical Volume read-only root file system, or
> > > alternatively let them share the bulk of the software (/usr, /opt, /lib)
> > > from a common KVM host Logical volume.
> > >
> > > Is this possible? All my searches so far failed to turn up anything like
> > > this.
> >
> > From the libvirt POV, there's nothing much todo except add <readonly/>
> > inside the <disk> element. This will ensure QEMU only gets given read
> > permission on the disk backend. The important thing is to then make sure
> > your guests actually mount the filesystem with the readonly flag.
> >
> > > Would it be possible using qcow2 instead of raw LV? If so - would it be
> > > worth the performance hit of switching from LV to qcow2?
> >
> > The type of backend storage doesn't really affect things if the
> > disk is fully readonly.
> >
> 
> Thanks very much! That's very helpful to know.
> 
> Is this something that someone has already done before (booting multiple
> KVM guests from shared read-only root file system) or am I on my own with
> this?

I've do it with libvirt-sandbox, but using a different approach. Instead
of a shared readonly disk image, I use the plan9 filesystem to pass a
chroot area from the host to the guest. Then I have to mount certain
areas with tmpfs or other writable filesystems (eg /var, /tmp/, parts
of /etc, etc).

What you want todo is definitely doable - the hard part is just in making
your OS image in a way to works with readonly root. This is typically not
to well documented, so involves alot of trial and error at first.

Daniel
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