On 21 September 2012 21:40, Daniel P. Berrange <berrange@redhat.com> wrote:

On Fri, Sep 21, 2012 at 09:34:02PM +1000, Amos Shapira wrote:
> On 21 September 2012 21:30, Daniel P. Berrange <berrange@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.

Thanks. I'll try to make my workplace interested in this and report back if/when I have interesting results.

In general, I'm thinking of replicating the way that diskless NFS clients used to boot from a common root file system on an NFS server, where the only difference among them is their MAC address, which in turn lets them pull a little different information (e.g. host name) from DHCP and use this as a basis to mount the private parts of the disk (e.g. /var, /etc?) and configuration.

The other question is whether I am correct in expecting that this could improve my disk IO profile, since all the guests would be accessing the same host's disk blocks.

Cheers,

--Amos

Daniel
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