[mrtg] Re: Different values in graphics

Steve Shipway s.shipway at auckland.ac.nz
Mon Mar 21 22:46:10 MET 2005

>My daily graphic has a 4Mb/sec peak but the weekly, monthly 
>and annual graphs shows different peak values. What I want is 
>the same 4Mb/sec peak value for all year,month,week,daily 
>graphs, how can I solve this? I tried the unscaled, gauge, 
>absolute options but nothing seem to help.

This is an unavoidable problem caused by the RRDTool rollup calculations and
statistics.  Basically, it is because:

Max[d](Avg[e](x)) != Max[e](x) for intervals d>e

Since, in a daily graph, the interval is 5min (one data point) you are
showing the max of this fine.  However, in a monthly graph, the interval is
2hours (24 data points) and the graph shows the average of these 24 data
points.  This smooths out the graph but results in a smoothed out Max line
as well.

The mathematics behind the calculations to explain the everage/max behaviour
as data is rolled up are relatively complex.  And as for trend

Hope this makes sense!


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