FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DC-TE, June 2003)

Planning Considerations for Fibre Channel Topologies
75SAN High Availability Planning Guide
■ Although the B_Port connection (ISL) between the director and switch is a
1.0625 Gbps serial connection, burst transmissions from multiple FC-AL
devices are multiplexed and buffered (the link BB_Credit value is eight), and
may coexist in the link. Therefore, the sum of the bandwidths of all devices
contending for B_Port access should not exceed 1.0625 Gbps. Exceeding the
total bandwidth may result in degraded performance, as shown in Figure 32.
Figure 32: ISL bandwidth limitation
Three 30-megabyte per second (MBps) tape drives and one 10-MBps server are
attached to a switch. Each device uses only a portion of the bandwidth of its
respective H_Port connection. Each H_Port connection illustrates four arbitration
cycles (each cycle won by a single device), one cycle of burst data transmission,
and three idle cycles. The B_Port connection to a director (ISL) transmits
multiplexed burst data from all four devices, for which the summed bandwidths
equal the 1.0625 Gbps capacity of the link. Therefore, connection of additional
devices to the switch adversely impacts B_Port performance.
(30 M ps)BD
1
S
1
(10 M ps)B
D
2
(30 M ps)B
D
3
(30 M ps)B
ISL (1 Gbps)
H_Ports
B_Port
Arbitration Cycle
Idle
Burst Data (D )
1
Burst Data (D )
2
Burst Data (D )
3
Burst Data (S )
1