I've stepped out of my comfort zone here. Some background. I just provisioned a 2nd vdisk+volume on a HP MSA2000. This SAN is has another vdisk+volume linked to Xenserver. I set the mapping to the hosts in the MSA config but when I try to add a new iSCSI SR in Xenserver and connect to any of the 4 (2x Port A, 2x Port B) iSCSI ips and select a IQN, it complains that the LUN is already in use by the other iSCSI SR. I even tried unmapping 2 of the ports from the existing vdisk/volume to see if freeing them up would help without success. I'm not even sure of the correct search phrase so be using so UTFSE has been useless. If anyone can point me in the right direction i'd be very grateful.
Ok I solved it. For those of you who might have found this thread the solution is: Create the vdisk and volume in the MSA2000 management interface and set all the ports EXPLICITLY mapped on both controllers ie A1, A2, B1, B2. If you try to add it now, Xenserver will complain, just reboot your pool master and the new LUN shows up without any complaining.