We just started setting up our first chassis/blade based server (Dell m1000e) which will host xen vms.

The physical OS (SLES11sp3) and xen virtual machines reside on the internal blade server drives (mirrored), but their data and mail (Novell NSS volumes) will live on our SAN, on a CLARiiON CX4.

This is still the testing stage, we haven't got any OES vms in the tree yet or NSS volumes to speak of, so I'm just working with straight linux and ext3 FS.

Environment:
I just created a couple of LUNs on the CX4 for testing.
So, I've got the physical server, with one LUN assigned/mounted directly to it (/dev/emcpowerb), and, one VM: I thought about using NPIV to get the vm to directly register to the CX4 but that seemed overly complicated. So, the physical box alone is zoned and registered to the CX-4, and all luns are seen by it, but any LUNs meant for a vm are simply added as add'l storage to the VM via xen. (and not mounted or used by the physical host). Make sense?

Situation:
I ran some tests writing a 3.2 GB iso file to the SAN luns on both phys and vm to compare performance; on the physical machine, it copies in about 5 seconds. Nice.
But on the vm, it either takes nearly a minute (!) or, it seems done in about 8 seconds but then any following command makes the machine hang at the prompt, even a simple command like "ls" or "hostname".

I have no idea what is causing this hanging, but it's not a good sign. The phys host seems okay, it's the vm that does this.

I don't really see anything in /var/log/messages that is useful. I know from the physical test that the SAN is good, it screams. But something seems amiss with xen.
Any ideas where to start to look?