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Fabric OS Administrator’s Guide 557
53-1002745-02
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.