Administrator Guide

Fabric OS Administrator’s Guide 137
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Buffer credit management
5
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, additional buffers are allocated to that port. Refer
to “Calculating the number of buffers required based on full-size frames” on page 138 and
“Configuring buffers for a single port directly” on page 141 for more information.
Fibre Channel gigabit values reference definition
Use the following Fibre Channel gigabit values to calculate buffer requirements:
Table 13 shows the Fibre Channel gigabit values used to calculate buffer requirements.
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 140 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 Fibre Channel gigabit values
Gigabit
value
Buffer requirements
1 Gbps 1.0625
2 Gbps 2.125
4 Gbps 4.25
8 Gbps 8.5
10 Gbps 10.625
16 Gbps 17