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Fibre Channel (SAN)

New Contributor
Posts: 3
Registered: ‎11-22-2017

buffer credit

dear all;

ihave a big conflict and misunderstanding how to calculate the number of buffer credits needs to be adjusted in brocade san director(DCX 8510) between  storage devices& hosts to acumelate the performance enhancments and to avoid any conjestion&delays  on ports ...and its the output for some ports


User     Port     Lx     Max/Resv  Avg Buffer Usage & FrameSize    Buffer Needed     Link     Remaining
Port     Type    Mode    Buffers        Tx         Rx              Usage  Buffers   Distance  Buffers
----     ----    ----    -------   ----------------------------    ------ -------   --------- ----------
  80               -         8       - (  - )      - (  - )           0       -          -
  81               -         8       - (  - )      - (  - )           0       -          -
  82       F       -        41       10(2012)       9(1912)          41       -          -
  83       F       -        41        4( 972)      15(3016)          41       -          -


so what is the recommendations also regarding the above outputs is it normal or upnormal


Broadcom Moderator
Posts: 108
Registered: ‎03-29-2010

Re: buffer credit

This is a very large, and possibly complex question. allocating buffer credits manually on local F ports is not recommended without a full understanding of how buffer credits work.


You may use the command 'portbuffercalc' with the speed, distance, and ave frame size to give you a buffer calculation. Note that for the average frame size, you will never, ever be able to fully subscribe all ports in a director to handle all frames without some congestion. There just aren't enough buffers in each ASIC to support every port until 100% utilization. Which is a good thing, because there is almost never a case where all ports on all ASICs are under full load.


Unless you are configuring ports for long distance, or you are seeing a Fabric Performance Impact from the MAPS  suite of management and monitoring, it's best to leave the buffer credit calculations up to the switch. If you begin to alter the buffer credits, note that other ports, not related to the port you are changing could suffer from lack of buffers, and cause unintended consequences.


Further reading on buffer credit allocation for FOS 7.x can be found here:


Beginning on page 119.


(The document library is under construction right now, so the links may change)


Any and all information provided by me is for entertainment value and should not be relied upon as a guaranteed solution or warranty of mechantability. All systems and all networks are different and unique. If you have a concern about data loss, or network disconnection, please open a TAC service request for service through Brocade, or through your OEM equipment provider. If this provided you with a solution to this issue, Please mark it with the button at the bottom "Accept as solution".

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