Users Guide

140 Fabric OS Administrator’s Guide

53-1002920-02

Buffer credit management

The following values are used in the example:

• 484 — The total number of unreserved buffer credits

• 206 — Buffer credits needed for 50 km at 8 Gbps

• 8 — The number of reserved buffer credits already allocated to that port

The resulting number is rounded down to the next whole number 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 1,024 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 137 for an approximation of the

calculated number of buffer credits.

1. Use the following formula to calculate the 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]

The average_payload_size in this equation uses 1024 bytes

If the real estimated distance is 100 km, the desired_distance is 207.

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. Refer to “Fibre Channel gigabit

values reference definition” on page 137 to determine the data_rate value.

For example, the data_rate is 8.5 for a speed of 8 Gbps.

4. Use the following formula to calculate the number of buffer credits to allocate:

buffer credits = [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 = [207 * (8.5 / 2.125)] = 828

NOTE

This buffer credits 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.