HP StorageWorks 8/80 HP StorageWorks Fabric OS 6.2 administrator guide (5697-0 - Page 427

19 Administering extended fabrics Licensing for Extended Fabrics An Extended Fabrics license is required before you can implement long distance dynamic (LD) and long distance static (LS) distance levels. The LD and LS settings are necessary to achieve maximum performance results over Inter-Switch Links (ISLs) that are greater than 10 km. Use the Time-Based Temporary Licensing that can be generated with the expiration date embedded in the license key to activate the Extended Fabrics feature. Use the licenseShow command to verify that the license is on both switches at either end of the extended ISL. For details on obtaining and installing licensed features, see "Licensed features" on page 43. Extended Fibre Channel over distance Extended Fabrics software optimizes switch buffering to ensure the highest possible performance on ISLs. When Extended Fabrics is installed on gateway switches, the ISLs (E_Ports) are configured with a large pool of buffer credits. The enhanced switch buffers help ensure that data transfer can occur at near-full bandwidth to efficiently utilize the connection over the extended links. The Extended Fabrics feature extends the distance the ISLs can reach over a dark fiber or wave division multiplexing (WDM) connection. This is accomplished by providing enough buffer credits on each side of the link to compensate for latency introduced by the extended distance. NOTE: Implementing extended fabrics between HP StorageWorks SAN Switch 2/8-EL, HP StorageWorks SAN Switch 2/16-EL, HP StorageWorks SAN Switch 2/16, and HP StorageWorks MSA SAN Switch 2/8; and switches running Fabric OS 6.x is not supported. Distance levels for extended Inter-Switch Links As the distance between switches and the link speed increases, additional buffer credits are required for the ports used for long-distance connections. Distance levels define how these buffer credits are allocated and managed for extended ISLs. Buffer credits are managed from a common pool available to a group of ports on a switch. The buffer credit can be changed for specific applications or operating environments, but it must be in agreement among all switches to allow formation of the fabric. Information about switch characteristics and capacity in terms of buffers per port group and distances supported are in Table 82 on page 427 and Table 83 on page 428. These tables may be useful in choosing an appropriate distance level, and for estimating the impact an extended ISL may have on buffer availability and port performance. Buffer-to-Buffer credits Buffer-to-Buffer (BB) credit flow control is implemented to limit the amount of data a port may send, based on the number and size of the frames sent from that port. Buffer credits represent finite physical port memory. Within a fabric, each port may have a different number of BB credits. Within a connection, each side may have a different number of BB credits. One buffer credit allows a device to send one payload up to 2112 bytes (2148 with headers). Assuming that each payload is 2112, you need one credit per 1 km of link length at 2 Gb/s (smaller payloads require additional BB credits to maintain link utilization). The final frame size must be a multiple of 4 bytes. If the data (payload) needs to segment, it will be padded with 1 to 3 "fill-bytes" to achieve an overall 4-byte frame alignment. The standard frame header size is 24 bytes. If applications require extensive control information, up to 64 additional bytes (for a total of an 88-byte header) can be included. Because the total frame size cannot exceed the maximum of 2148 bytes, the additional header bytes will subtract from the data segment size by as much as 64 bytes (per frame). This is why the maximum data (payload) size is 2112 (because [2112 - 64] = 2048, which is exactly 2 Kbytes of data). The final frame, after it is constructed, is passed through the 8-byte to 10-byte conversion process. Fabric OS 6.2 administrator guide 423