HP StorageWorks 2/24 FW 07.00.00/HAFM SW 08.06.00 McDATA Products in a SAN Env - Page 109

Planning Considerations for Fibre Channel Topologies 3 • Preferred path - Preferred path is an option that allows a user to configure an ISL data path between multiple fabric elements (directors and fabric switches) by configuring the source and exit ports of the origination fabric element and the Domain_ID of the destination fabric element. Each participating director or switch must be configured as part of a desired path. For information about the feature, refer to Preferred Path. ATTENTION ! Activating a preferred path can result in receipt of out-oforder frames if the preferred path differs from the current path, if input and output (I/O) is active from the source port, and if congestion is present on the current path. In general, Fibre Channel frames are routed through fabric paths that implement the minimum possible hop count. For example, in Figure 3-11, all traffic between devices connected to director S1 and director S2 communicate directly through ISLs that connect the directors (one hop). No traffic is routed through director S3 (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. Special consideration must also be given to applications with high data transfer rates or devices that participate in frequent or critical data transfer operations. For example, in Figure 3-11, suppose device D7 is a server and device D9 is a storage unit and both devices participate in a critical nightly backup operation. It is recommended that such a connection be routed directly through director S2 (rather than the entire fabric) through zoned port connections, WWN-bound port connections, or a preferred path. For additional information, refer to Device Locality on page 3-34. Planning Considerations for Fibre Channel Topologies 3-23