Dell Brocade G620 Brocade 8.0.1 Fabric OS Administratiors Guide - Page 131
Allocating buffer credits based on average-size frames, Maximum_Distance_X
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Buffer-to-Buffer Credits and Credit Recovery 676 - (24 * 8) = 484 unreserved buffer credits 492 buffers to a single port (484 + 8 [8 for the reserved buffers already allocated to that user port]), you can calculate the maximum single-port extended distance supported: Maximum_Distance_X (in km) = (BufferCredits + 6) * 2 / LinkSpeed 498 km = (492 + 6 buffers for Fabric Services) * 2 / 2 Gbps If you have a distance of 50 km at 8 Gbps, then 484 / (206 - 8) = 2 ports. 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 128 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 128 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. Brocade Fabric OS Administration Guide, 8.0.1 53-1004111-02 131
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