Allocating, Buffer credits based on average-size frames – Dell POWEREDGE M1000E User Manual
Page 489
Fabric OS Administrator’s Guide
449
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Buffer credit management
20
24 = the number of user ports in a port group retrieved from
8 = the number of reserved credits for each user port.
676 = the number of buffer credits available in the port group.
If you allocate the entire 484 + 8 (8 for the reserved buffers already allocated to that user port)
= 492 buffers to a single 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
How many 50 km ports can you configure?
If you have a distance of 50 km at 8 Gbps then, 484 / (206 – 8) = 2 ports
The numbers used are: 484, which equals the total number of unreserved buffer credits, 206,
which equals buffer credits needed for 50 km @ 8 Gbps (calculated previously), and 8, which
equals 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 give twice the distance in the long-distance LS configuration mode. Refer to the
Fibre Channel gigabit values reference definition
to get an approximation of the calculated number
of buffer credits.
1. Use the following formula to calculate value for the desired_distance needed for Fabric OS to
determine the number of BB 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 BB credits.
2. Determine the speed that 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
on page 447 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-to-buffer credits to allocate:
BB credits = roundup [desired_distance * (data_rate / 2.125)]
Using the values for desired_distance and data_rate from
, the value for BB
credits is calculated as follows:
BB credits = roundup [(207 * 8.5) / 2.125] = 828