Dell Brocade G620 Brocade 8.0.1 Fabric OS Administratiors Guide - Page 129

Buffer-to-Buffer Credits and Credit Recovery TABLE 12 Fibre Channel gigabit values (continued) Gigabit value 16 Gbps 32 Gbps Line rate 17 28.05 Buffer credit allocation based on full-size frames Assuming that the frame is a full-size frame, one buffer credit allows a device to send one payload up to 2,112 bytes (2,148 with headers). Assuming that each payload is 2,112, you need one credit per 1 km of link length at 2 Gbps (smaller payloads require additional buffer credits to maintain link utilization). Refer to Allocating buffer credits based on average-size frames on page 131 for additional information. Fibre Channel data frames The final frame size must be a multiple of 4 bytes. If the data (payload) needs to be segmented, 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 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 kb of data). The final frame, after it is constructed, is passed through the 8-byte-to-10-byte conversion process. Table 13 describes Fibre Channel data frames. TABLE 13 Fibre Channel data frames Fibre Channel frame fields Start of frame Standard frame header Data (payload) CRC End of frame Total (number bits/frame) Field size 4 bytes 24 bytes 0-2,112 bytes 4 bytes 4 bytes 36-2,148 bytes Final frame size 32 bits 192 bits 0-16,896 bits 32 bits 32 bits 288-7,184 bits Allocating buffer credits based on full-sized frames 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 mode (LS) to calculate the buffer 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. A best practice is to use LS over LD. The assumption that Fibre Channel payloads are consistently 2,112 bytes is not realistic in practice. To gain the proper number of buffer credits with the LS mode, there must be enough buffer credits available in the pool, because Fabric OS will check before accepting a value. NOTE The desired_distance parameter of the portCfgLongDistance command's is the upper limit of the link distance and is used to calculate buffer availability for other ports in the same port group. When the measured distance exceeds the value of desired_distance, this value is used to allocate the buffers. In this case, the port operates in degraded mode instead of being disabled as a result of insufficient buffer availability. In LS mode, the actual link distance is not measured; instead, the desired_distance value is used to allocate the buffers required for the port. Brocade Fabric OS Administration Guide, 8.0.1 53-1004111-02 129