IBM社のサイトより、(自己責任)ですが、非常に参考になります。

Citrix XenServer技術情報
http://www-06.ibm.com/jp/domino04/pc/support/Sylphd07.nsf/jtechinfo/SYJ0-027B98E

Device-Mapper MultipathによるSAN Boot環境構築 (RHEL5.1)
http://www-06.ibm.com/jp/domino01/mkt/cnpages7.nsf/page/default-002673C7

Citrix XenServer 5.0.0 検証レポート – FC Storage編 –
http://www-06.ibm.com/jp/domino01/mkt/cnpages7.nsf/page/default-0033B0AA

After working with IBM, we learned that ds4xxx are OEM from LSI Logic (like MD3000 from Dell), thus RDAC driver (mppxxxx modules provided from LSI through Dell, included in XS5 release) is loaded automatically, and can handle multipathing without enabling multipathd(no need to edit multipath-enabled.conf).

If you still want to configure multipathing thru multipath.conf, you have to disable loading mppxxx, but it was possible by manually altering initrd/init, as xs5 loads mppxxx earlier than mptsas/mptscsih, and the device nodes are already handled(modledep() hard-coded in /sbin/mkinitrd script).

With RDAC(mppxxx), you can track the status in  /proc/scsi/mpp/n.
You must set the hosttype to LNXCLVMWARE when RDAC is used(RDAC sends failover/failback commands directly to controllers).

We tested in IBM, all worked fine without any issue.

We tested in customer’s datacenter, the main concern were resolved, multipathing working fine(failover, failback), although a minor issue was found (pdb is unplugged at boot – but now possible to plug it manually after boot, so pbd-plug has been added to the rc.local with sleep to work around), this may be specific to cutomer’s enviroment.