[mrtg] Strange MRTG problem

[Steve Shipway]

>(Cacti is running on 1 minute, MRTG is running on 10 minutes.)

And here we have the explanation.

If MRTG is only polling every 10min (and you have the wrong Interval in the cfg file and in your RRD file for this) then it will be, in effect, giving you a 10min average speed for the port.

Cacti, on the other hand, is polling every 1min and so is giving you 1min averages.

Therefore, the MRTG graph will be much more smoothed out and flatter, and Cacti's graph will be more spiky, showing higher peaks for bursts of traffic that are relevant in a 1min interval but not over a 10min interval.  This is why you see much higher values in Cacti.  The same effect happens in MRTG if you compare the peaks on a 5min average graph (daily) and on the 2hour average graph (monthly).

Consider this set of 1-minute interval data:

10
1
1
1
1
1
1
1
1
1

This is a 10min sample; for MRTG it would see a total of 19, IE 1.9/min.   Cacti, however, would see them all, and show a graph with a spike up to 10.  Therefore, very different graphs.

If you want them to match, you will need to change the MRTG Interval to 1 minute, recreate your RRD files, and poll every minute - and change your PHP graphing script to take account of the higher granularity of data available.

Steve

Steve Shipway
University of Auckland ITS
UNIX Systems Design Lead
s.shipway at auckland.ac.nz<mailto:s.shipway at auckland.ac.nz>
Ph: +64 9 373 7599 ext 86487
Voice: (414) 810-6392
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