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

On Fri, Sep 21, 2012 at 09:34:02PM +1000, Amos Shapira wrote: > On 21 September 2012 21:30, Daniel P. Berrange <berrange(a)redhat.com&gt; 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 -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/:| |: http://libvirt.org -o- http://virt-manager.org:| |: http://autobuild.org -o- http://search.cpan.org/~danberr/:| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc:|