Hi Andy
see eg.
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/5_2/configuration/guides/fund/DCNM-SAN-LAN_5_2/DCNM_Fundamentals/perfmon.html
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/5_2/configuration/guides/fund/DCNM-SAN-LAN_5_2/DCNM_Fundamentals/perfmon.html#wp1306714
Walter.