Dell PowerConnect Brocade 300 Fabric OS Administrator's Guide v7.1.0 - Page 557

Buffer credit management 23 Smaller frame sizes need more buffer credits. Two commands are available to help you determine whether you need to allocate more buffer credits to handle the average frame size. The portBufferShow command calculates the average frames size. The portBufferCalc command uses the average frame size with the speed and link distance to determine the number of buffer credits needed. Considerations for calculating buffer credits Considerations follow for calculating how many ports can be configured for long distance on all Fabric OS v7.x-capable switch modules: • Each port is part of a port group that includes a pool of buffer credits that can be used. This port group is not the same as the port groups used for ISL Trunking. • Each user port reserves eight buffer credits when online or offline. • Any remaining buffers can be reserved by any port in the port group. • When QoS is enabled and the port is online, an additional 20 buffers are allocated to that port. Fibre Channel gigabit values reference definition Use the following Fibre Channel gigabit values to calculate buffer requirements: • 1.0625 for 1 Gbps • 2.125 for 2 Gbps • 4.25 for 4 Gbps • 8.5 for 8 Gbps • 10.625 for 10 Gbps • 17 for 16 Gbps 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 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 buffer credits to maintain link utilization). See "Allocating buffer credits based on average-size frames" on page 560 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 kbs of data). The final frame, after it is constructed, is passed through the 8-byte-to-10-byte conversion process. Table 82 describes Fibre Channel data frames. Fabric OS Administrator's Guide 557 53-1002745-02