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Occasional Contributor
Posts: 14
Registered: ‎12-23-2012

Average Frame Size for Buffer Credit calculation - which one to use Tx frame size or Rx frame size ??

Hi There

 

portbuffershow command gives the average Tx and average Rx frame size . 

 

Which one should we take for buffer credit calculation  ??

 

 

Thanks in Advance 

 

Anand

Brocade Moderator
Posts: 170
Registered: ‎03-29-2011

Re: Average Frame Size for Buffer Credit calculation - which one to use Tx frame size or Rx frame size ??

I would use both, take the minimum. And also to own calculation of the frame size based on

 

  1. clear the stats on the ports - portstatsclear <port>
  2. wait maybe 2h
  3. save the output from portstats64show <port> -long for that port
  4. calculate the average send frame size by

 

4 * stat64_wtx / stat64_ftx = average frame size in bytes for transmitted frames

4 * stat64_wrx / stat64_frx = average frame size in bytes for received frames

 

 


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Brocadian
Posts: 63
Registered: ‎04-16-2010

Re: Average Frame Size for Buffer Credit calculation - which one to use Tx frame size or Rx frame size ??

The configured buffer credits on the switch are what we advertise as available credits so it is relevant to Rx buffers. Typically, BB credits are only adjusted on E-Ports (ISLs). Since Tx buffers on one end of the link is the same as Rx buffers on the remote end, you can use the Tx buffer information on the local switch to be representative of what the remote side is recieving.

 

Assuming you are interested in determining the right number of BB credits for an ISL, if traffic is truley bi-directional use the smallest frame size to ensure adequate BB credits to keep the link full. Often, traffic is really uni-directional. For example, tape backups almost always go from a production site to a backup site. In this example, there will be a handful of frames from the backup site to the production site. Those frames are generally small since they are usually carrying status information and not user data. This type of data is limited to a small number of frames not requiring much B/W so using this data in your calculation wouldn't make sense.

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