426 Administering extended fabrics
1. Determine the desired distance in kilometers between 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 Gb/s.
3. Use the following formula to calculate the reserved buffers for distance:
(Reserved Buffer for Distance Y) = (X * LinkSpeed / 2) + 6
Where:
X = the distance determined in step 1 (in kilometers).
LinkSpeed = the speed of the link determined in step 2.
6 = the number of buffer credits reserved for Fabric Services, Multicast, and Broadcast traffic. This is
a static number.
Based on the answers provided in steps 1 and 2, insert the numbers into the formula. The formula
should read as follows:
(50 km * 2 Gb/s / 2) + 6 = 56 buffers, which is the number of buffers reserved for distance
Below are additional examples using different speeds all based on a distance of 50 km. The distances and
speeds are variables that can change based on how your network is set up.
• If you have a distance of 50 km at 1 Gb/s, then,(50 km * 1 Gb/s / 2) + 6 = 31 buffers
• If you have a distance of 50 km at 2 Gb/s, then (50 km * 2 Gb/s / 2) + 6 = 56 buffers
• If you have a distance of 50 km at 4Gb/s, then,(50 km * 4 Gb/s/ 2) + 6 = 106 buffers
• If you have a distance of 50 km at 8 Gb/s, then (50 km * 8 Gb/s/ 2) + 6 = 206 buffers
Example: Consider the HP StorageWorks 8/8 and 8/24 SAN Switches, which have a single 24-port port
group and a total of 676 buffer credits for that port group
The maximum remaining number of buffer credits for the port group, after each port reserves its eight buffer
credits is:
676 – (24 * 8) = 484 unreserved buffer credits
Where:
24 = the number of user ports in a port group retrieved from Table 82 on page 427.
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 Gb/s
How many 50 km ports can you configure?
If you have a distance of 50 km at 8 Gb/s, 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 Gb/s (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 Gb/s, 484 / (31 – 8) = 21 ports
Displaying the remaining buffers in a port group
Enter the portBufferShow command to display all of the long-distance buffer information for all the port
groups of the switch. Use the following syntax:
portbuffershow [[slotnumber/]portnumber]
slotnumber For bladed systems only, specifies the slot number of the port group to
display, followed by a slash (/).
portnumber Specifies the number of a port associated with the port group, relative to its
slot for bladed systems. Use the switchShow command to display a list of
valid ports.
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