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560 Fabric OS Administrator’s Guide
53-1002745-02
Buffer credit management
23
• 8 — the number of reserved buffer credits already allocated to that port.
The floor of the resulting number is taken because fractions of a port are not allowed.
If you have a distance of 50 km at 1 Gbps, then 484 / (31 – 8) = 21 ports
Allocating buffer credits based on average-size frames
In cases where the frame size is average, for example 1024 bytes, you must allocate twice the
buffer credits or configure twice the distance in the long-distance LS configuration mode. Refer to
“Fibre Channel gigabit values reference definition” on page 557 for an approximation of the
calculated number of buffer credits.
1. Use the following formula to calculate value for the desired_distance parameter needed for
Fabric OS to determine the number of buffer credits to allocate:
desired_distance = roundup [(real_estimated_distance * 2112) / average_payload_size]
Where average_payload_size = 1024 bytes
This example uses 100 km for the real estimated distance.
desired_distance = roundup [(100 * 2112) / 1024] = 207
When configuring the LS mode with the portCfgLongDistance command, enter a
desired_distance value of 207 for an actual 100-km link connected to an 8-Gbps E_Port. This
causes Fabric OS to allocate the correct number of buffer credits.
2. Determine the speed you will use for the long-distance connection. This example uses 8 Gbps.
3. Look up the data_rate value for the speed of the connection. See “Fibre Channel gigabit values
reference definition” on page 557 to determine the data_rate value.
For 8 Gbps, the data_rate is 8.5
4. Use the following formula to calculate the number of buffer credits to allocate:
buffer credits = roundup [desired_distance * (data_rate / 2.125)]
With the values for desired_distance and data_rate from step 1 and step 3, the value for buffer
credits is calculated as follows:
buffer credits = roundup [(207 * 8.5) / 2.125] = 828
NOTE
This formula does not work with LD mode because LD mode checks the distance and limits the
estimated distance to the real value of 100 km. LS mode allows for the necessary desired_distance
value based on the data size entered, regardless of the distance.
If buffer credit recovery is enabled, Fabric OS supports a BB_SC_N range of 1 to 15; therefore, it is
impossible for the desired_distance value to be more than the number of buffer credits available in
the pool as determined by the calculations above. The distance for buffer credit recovery is well
within the range of all possible connections. An estimated distance of 32,768 is considerably
higher than the available buffer credits and only lower values of desired_distance are permitted by
Fabric OS.