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

[Amos Shapira]

On 21 September 2012 21:40, Daniel P. Berrange <berrange at 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 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.
>

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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