Dell POWEREDGE M1000E - Manual (Page 558)

558 Fabric OS Administrator’s Guide
53-1002745-02
Buffer credit management
23
Allocating buffer credits based on full-sized frames
You can allocate buffer credits based on distance using the portCfgLongDistance command. The
long-distance link modes allow you to select the dynamic mode (LD) or the static mode (LS) to
calculate the buffer credits.
For LD, the estimated distance in kilometers is the smaller of the distance measured during port
initialization versus the desired_distance parameter, which is required when a port is configured as
an LD or an LS mode link. It is best practice to use LS over LD. The assumption that Fibre Channel
payloads are consistently 2,112 bytes is not realistic in practice. To gain the proper number of
buffer credits with the LS mode, there must be enough buffer credits available in the pool, because
Fabric OS will check before accepting a value.
NOTE
The portCfgLongDistance command’s desired_distance parameter is the upper limit of the link
distance and is used to calculate buffer availability for other ports in the same port group. When the
measured distance exceeds the value of desired_distance, this value is used to allocate the buffers.
In this case, the port operates in degraded mode instead of being disabled asa result of insufficient
buffers. In LS mode, the actual link distance is not measured; instead, the desired_distance value
is used to allocate the buffers required for the port.
Refer to the data in Table 83 on page 563 and Table 84 on page 564 to get the total ports in a
switch or blade, the number of user ports in a port group, and the unreserved buffer credits
available per port group. The values reflect an estimate, and may differ from the supported values
in
Table 84 .
Calculating the number of buffers required based on full-size frames
Use the following procedure to calculate the number of buffers required for a long-distance
connection:
1. Determine the desired distance in kilometers of the switch-to-switch connection. This example
uses 50 km.
2. Determine the speed that you will use for the long-distance connection. This example uses 2
Gbps.
3. Use one of the following formulas to calculate the reserved buffers for distance:
• If QoS is enabled:
(Reserved Buffer for Distance Y) = (X * LinkSpeed / 2) + 6 + 14
TABLE 82 Fibre Channel data frames
Fibre Channel frame fields Field size
Start of frame 4 bytes 32 bits
Standard frame header 24 bytes 192 bits
Data (payload) 0–2,112 bytes 0–16,896 bits
CRC 4 bytes 32 bits
End of frame 4 bytes 32 bits
Total (number bits/frame) 36–2,148 bytes 288–7,184 bits