On 21 September 2012 21:40, Daniel P. Berrange <berra...@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 <berra...@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 > -- > |: 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:| > -- [image: View my profile on LinkedIn] <http://www.linkedin.com/in/gliderflyer>
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