How to determine the average frame size
seb_ 060000QVK2 Comments (3) Visits (19210)
I was asked where to look at in a switch to find the average frame size for a port. The safest way would be to use an external monitoring tool like a VirtualWisdom or a tracer as described in my LD mode article but if you don't own something like that you can get a good guess from the switches themselves. You just have to calculate it out of the number of frames and the number of bytes transferred.
Cisco MDSFor Cisco it's easy. Just look into the "show interface" for the specific port and you'll find the both numbers in the statistics section for each interface:
1887012 frames input, 1300631486 bytes ... 542470 frames output, 482780325 bytes
So we can just calculate the average frame sizes for both directions:
1300631486 bytes / 1887012 frames = 689 bytes per frame
For Brocade switches you can get the information out of the portstatsshow command:
stat_wtx 35481072 4-byte words transmitted stat_wrx 70173758 4-byte words received stat_ftx 1111087 Frames transmitted stat_frx 1177665 Frames received
Here we don't have the plain bytes but 4-byte words. Don't worry - fillwords don't count into this number, so it's still valid for our calculation. We just have to multiply it by four to use it:
(35481072 * 4) bytes / 1111087 frames = 128 bytes per frame
It's really that easy?
Basically yes. With this average frame size you can find out the multiplier for the buffer credits settings. So if you have an average frame size of 520 and a link of 30 km, just calculate:
2112 (the max frame size) / 520 = 4
So you would set up the link for 120 km instead of 30 km to reserve a sufficient amount of buffers. That's it.
A last catch
If you read my article about bottleneckmon you probably already know that we work with 32 bit counters here. While they cover a few hours for the frames they wrap much quicker for the 4-byte words. So to be able to calculate an average frame size over several hours or days, 32 bit counters are not enough. Actually there are 64 bit counters for these values in the switches - although they are not part of a supportsave. The command portstats64show provides them. The first thing to keep in mind: While in the latest FabOS versions a statsclear resets these counters as well, you had to reset them with portstatsclear in the older versions.
The 64 bit counters are actually two 32 bit counters and the lower one ("bottom_int") is the 32 bit counter we used all the time in portstatsshow. But each time it wraps, it increases the upper one ("top_int") by 1. So after a while you might see an portstats64show output like this:
stat64_wtx 0 top_int : 4-byte words transmitted 2308091032 bottom_int : 4-byte words transmitted stat64_wrx 39 top_int : 4-byte words received 1398223743 bottom_int : 4-byte words received stat64_ftx 0 top_int : Frames transmitted 9567522 bottom_int : Frames transmitted stat64_frx 0 top_int : Frames received 745125912 bottom_int : Frames received
For the received frames it's then:
(2^32 * 39 + 1398223743) * 4 bytes / 745125912 frames = 907 bytes per frame.
Much manual computing, hmm?
Of course you could write a script for that or prepare a spreadsheet but my recommendation is still to start with a multiplier of 3 for normal open systems traffic and check with the command portbuffershow how many buffers are still available. And if you still have some, use them - but keep them in mind if you connect additional long distance ISLs or devices you want to give additional buffers as well.
Update Nov. 2nd 2012:
I was made aware that there is an easier and much more convenient way to use portstats64show: Just use the -long option.
Much better, isn't it? Thanks to Martin Lonkwitz!