HP StorageWorks 64 - SAN Director Switch Planning Manual page 69

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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
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.
Planning Considerations for Fibre Channel Topologies
69