On Fri, Sep 21, 2012 at 09:48:58PM +1000, Amos Shapira wrote: > On 21 September 2012 21:40, Daniel P. Berrange <berrange@xxxxxxxxxx> wrote: > > > On Fri, Sep 21, 2012 at 09:34:02PM +1000, Amos Shapira wrote: > > > On 21 September 2012 21:30, Daniel P. Berrange <berrange@xxxxxxxxxx> > > 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. The normal KVM recommended practice is to use cache=none and let the guest OS cache I/O on the basis that you can then directly control guest memory usage. This is probably the exact opposite of what you want though. To maximise performance I expect you want to let the host OS aggressively cache the shared disk image (cache=writethrough/unsafe), and limit (but not eliminate) the amount of extra caching the guest does 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 :| _______________________________________________ libvirt-users mailing list libvirt-users@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvirt-users
