HP Surestore 64 FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availabil - Page 69
Number of loop tenancies, Service rate, Loop utilization
View all HP Surestore 64 manuals
Add to My Manuals
Save this manual to your list of manuals |
Page 69 highlights
Planning Considerations for Fibre Channel Topologies ■ Number of loop tenancies - Each cycle of device arbitration, loop opening, frame transmission, frame reception, and loop closing is called a loop tenancy. A Fibre Channel operation, such as a small computer system interface (SCSI) write command, may require several tenancies to complete. Because significant overhead is associated with establishing and ending each loop tenancy, an increase in tenancies decreases loop performance. To decrease the number of loop tenancies, plan to limit the number of arbitration-initiating devices installed on the loop. ■ Service rate - The loop service rate is the number of H_Port service requests the arbitrated loop can process in a time period, and is defined as one divided by the average loop tenancy duration. Long-duration loop tenancies decrease the loop service rate because select devices monopolize the loop. High-bandwidth storage devices that can rapidly process input/output (I/O) requests typically cause long-duration loop tenancies. Plan to limit the number of such devices installed on the loop. ■ Loop utilization - Loop utilization is the term that describes how busy an arbitrated loop is, and is defined as the request rate divided by the service rate. The request rate is the rate at which devices arbitrate for access to the loop, and is a function of applications using the loop, not the loop itself. As the request rate increases due to additional devices being added to the loop, the probability of contention for loop access and arbitration wait time increases. In fact, as loop utilization increases, arbitration wait time increases nonlinearly. Shared mode operation does not fully use the switch's capabilities and should be used only when connecting legacy FC-AL devices that do not support switched mode operation. Although the architectural limit of a Fibre Channel arbitrated loop is 125 devices, 32 or fewer devices should be attached to the switch to avoid adversely impacting loop performance. In particular, avoid attaching an excess number of servers or high-bandwidth storage devices. SAN High Availability Planning Guide 69