Just so I know I'm understanding this all correctly.... Xen allows someone to have many physical machines running many virtual machines? So, hypothetically, you can have 15 virtual machines that all share 5 physical machines---allowing you to hot swap or add physical hosts to the Xen cluster without downtime to the virtual hosts?

More or less.

In order to do that, Xen has a feature called "live migration", but it requires that the disk images (or partitions) are on shared/networked storage. As long as you have the networked storage, you could move all Xen domains off a machine onto the others, take that machine down for maintenance or whatever, and then move domains back to it once it's back online...

Xen also supports suspend-to-disk, so one could suspend a Xen domain, copy the resulting Xen "image" along with the disk images to another machine, and then restore it back to a running state there...

HTH,
-Chris

I think what I'm needing the most clarification on is this: Are the collective resources of 3 physical machines really able to be shared by a group of virtual machines? As UML hosting goes usually, you have a single physical machine serving a group of virtual hosts--will Xen allow you have several virtual machines being served by a cluster (as opposed to a single) physical machine? Could one concievably pool resources of several powerful physical machines to serve one virtual machine with an increase of performance?

Yes. Using live-migration (and as long as you have networked storage) you could load-balance your running domains across those three machines. But, Xen doesn't do this automatically -- you'd need to script up a load-balancer to initiate moving the domains around.


Kinda -- See answer above.


No. A Xen domain runs on top of one host, and one host only. It can be moved to another host, but it's still limited by the resources of a single machine.

-Chris

Thanks for clearing that up... That would be incredible technology, though

Erik:

What you describe at the end of your post is basically the premise behind single system image clustering.

Ever heard of OpenMOSIX?

http://openmosix.sourceforge.net/

This is way up there on my "fun things to do when I have a whole computer lab to myself" list.

The really interesting thing about OpenMOSIX is that it runs unmodified applications. You don't need to use a specialized framework like MPI or PVM to run across multiple nodes.

(other references)
http://www-unix.mcs.anl.gov/mpi/mpich/
http://www.csm.ornl.gov/pvm/
http://www.beowulf.org/


--Brandon

_________________
--Bdragon

This is now completly off topic, but what the hell.

I couldn't see a mention of OpenMosix without mentioning ClusterKnoppix - http://clusterknoppix.sw.be/ Makes it trivial to play around with this stuff - 1 cd and a hijacked computer lab are all you need!