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

[Amos Shapira]

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?

Cheers,

--Amos
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