HP StorageWorks 1606 Brocade Fabric OS Administrator's Guide v6.3.0 (53-100133 - Page 493

Buffer credit management 20 Fibre Channel gigabit values reference definition Before you can calculate the buffer requirement, note the following Fibre Channel gigabit values reference definition: • 1.0625 for 1 Gbps • 2.125 for 2 Gbps • 4.25 for 4 Gbps • 8.5 for 8 Gbps Allocating buffer credits based on full-size frames Assuming that the frame size is full, 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 Gbps (smaller payloads require additional BB credits to maintain link utilization). For information on allocating buffer credits on average size frames, see "Allocating buffer credits based on average-size frames" on page 453 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 2,148 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 2,112 (because [2,112 - 64] = 2,048, which is 2 kbs of data). The final frame, after it is constructed, is passed through the 8-byte to 10-byte conversion process. The following table describes Fibre Channel data frames. TABLE 87 Fibre Channel data frames Fibre Channel Frame fields Field size Start of frame 4 bytes Standard frame header 24 bytes Data (payload) 0 - 2,112 bytes 32 bits 192 bits 0 - 16,896 bits CRC 4 bytes 32 bits End of frame 4 bytes 32 bits Total (Nbr bits/frame) 36 - 2,148 bytes 288 - 17,184 bits You can allocate buffer credits based on distance using the portCfgLongDistance command. The Long distance link modes allow you to select the Dynamic mode (LD) or the Static Long-distance mode (LS) to calculate the BB credits. For LD, the estimated distance in kilometers is the smaller of the distance measured during port initialization versus the desired_distance parameter, which is required when a port is configured as an LD or an LS mode link. It is best practice to use LS over LD. The assumption of Fibre Channel payloads consistently being 2,112 bytes is not realistic in practice. To gain the proper number of BB credits using the LS mode, there must be enough BB credits available in the pool because Fabric OS will check before accepting a value. Fabric OS Administrator's Guide 451 53-1001336-01