libvirt users - Sep 2012 - sharing read-only LOCAL disks among KVM guests

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.

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 motivation for this is two-fold:
1. Speed-up - I suspect that sharing the same disk block among all KVM
agents (we currently have 14 KVM guests on each server) should lead to
better cache utilisation and reduce disk access. Also the cost of
maintaining the mutiple identical servers and keeping them up to date could
be eating into our server's resource usage.
2. Maintenance - if I get to share the common disk image among all of them
then I only need to install/update it once instead of once for each KVM
guest (we already use Puppet but it doesn't mean it comes for free).

Thanks,

--Amos
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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.

Daniel
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On Fri, Sep 21, 2012 at 09:22:24PM +1000, Amos Shapira
wrote:
> 2. Maintenance - if I get to share the common disk image among all of them
> then I only need to install/update it once instead of once for each KVM
> guest (we already use Puppet but it doesn't mean it comes for free).
How do you plan on upgrading the read-only VMs after the initial
installation, or are they short-lived and frequently recreated from
scratch using a fresh image? Read-only, shared /usr between machines
is a nice idea in theory, but at least the modern linux distros'
package management tend not to support it that well in practice.

IMHO using a QCOW2 backing files as base templates works pretty well
and gives you similar disk space savings, at least initially. And
KSM does the same with RAM.