Configuring buffers for a single port directly, 120 and, Configuring – Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) User Manual
Page 122: Buffers for a single port directly
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
on page 119 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.
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 previous calculations 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.
Configuring buffers for a single port directly
To configure the number of buffers directly, use the -buffers option of the portCfgLongDistance
command. Fabric OS uses this value to calculate the total number of buffers according to the following
formula:
Total Buffers = Configured Buffers + QOS_VC_Credits + Non-data_VC_Credits
Seven Virtual Channels (VCs) are required for each QoS port. Each VC requires two buffers. Thus,
the total number of QoS buffers required for a port is 14 (7*2). An additional 6 VCs are required for
nondata transmission (for example, control traffic). As a consequence, for a QoS port, 20 buffers are
added. For a non-QoS port, 6 buffers are added.
For example, if the configured number of buffers is 100, then the total number of buffers allocated for a
QoS port is 120, as shown in the following example.
Total_Buffers = 100 + 14 + 6 = 120
If the configured number of buffers is 100, the total number of buffers allocated for a non-QoS port is
106, as shown in the following example.
Total_Buffers = 100 + 6 = 106
Configuring buffers for a single port directly
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Fabric OS Administrators Guide
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