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
82 SAN High Availability Planning Guide
■ Load balancing — Planning consideration must be given to the amount of
data traffic expected through the fabric or through a fabric element. Because
the fabric automatically determines and uses the least cost (shortest) data
transfer path between source and destination ports, some ISL connections
may provide insufficient bandwidth while the bandwidth of other connections
is unused. For additional information, refer to “Large Fabric Design
Implications” on page 107.
Fibre Channel frames are routed through fabric paths that implement the
minimum possible hop count. For example, in Figure 35 on page 78, all traffic
between devices connected to Director S
1
and Director S
2
communicates
directly through ISLs that connect the directors (one hop). No traffic is routed
through Director S
3
(two hops). If heavy traffic between the devices is
expected, multiple ISL connections should be configured to create multiple
minimum-hop paths. With multiple paths, the directors balance the load by
assigning traffic from different ports to different minimum-hop paths (ISLs).
When balancing a load across multiple ISLs, a director or switch attempts to
avoid assigning multiple ports attached to a device to the same ISL. This
minimizes the probability that failure of a single ISL will affect all paths to the
device. However, because port assignments are made incrementally as devices
log into the fabric and ISLs become available, optimal results are not
guaranteed.
Table 2: ISL Transfer Rate versus Fabric Port Availability (Two-Director Fabric)
Number
of ISLs
ISL Data Transfer
Rate (at 1.0625 Gbps)
ISL Data Transfer Rate
(at 2.125 Gbps)
Available
Fabric Ports
1 1.0625 Gbps 2.1250 Gbps 126
2 2.1250 Gbps 4.2500 Gbps 124
3 3.1875 Gbps 6.3750 Gbps 122
4 4.2500 Gbps 8.5000 Gbps 120
5 5.3125 Gbps 10.6250 Gbps 118
6 6.3750 Gbps 12.7500 Gbps 116
7 7.4375 Gbps 14.8750 Gbps 114
8 8.5000 Gbps 17.0000 Gbps 112