I've never seen this posted anywhere, and after going through great pain with getting this configured and setup I figured that it would be good to share the wealth.
When setting up a Qlogic FC switch between a MDC and the Xserve RAID, it's very important to configure the switches as follows:
RAID attached ports (meaning coming to or from the FC switch and the RAID system) Set the port to use "F" for Fabric and make sure the I/O stream guard is disabled.
XServe attached ports (meaning the cable going to and from the FC switch and the Xserve), Set the port to use "F" for Fabric and make sure the I/O stream guard is enabled.
Without the ports set this way it caused us a great deal of headaches with the system dumping the FC network when the traffic patterns got to high. Once we had this set, with the aid of an Apple engineer, we were able to use the Xsan Tuner to test the I/O connection and it appeared fine.
Good luck!
Brian