[rrd-users] VDEF MINIMUM

Alex van den Bogaerdt alex at vandenbogaerdt.nl
Tue Aug 9 15:00:58 CEST 2011

> I'm seeing more marked differences in VDEF MINIMUM now.
> m at n:~$ rrdtool graph testing.png -s end-10000
> DEF:noise=noise.rrd:noise:AVERAGE VDEF:min=noise,MINIMUM PRINT:min:%.1lf
> 0x0
> 5.2
> m at n:~$ rrdtool graph testing.png -s end-25000
> DEF:noise=noise.rrd:noise:AVERAGE VDEF:min=noise,MINIMUM PRINT:min:%.1lf
> 0x0
> 6.9

You are looking at averages. Your two tests use different RRAs. For
instance: one RRA consolidates 1 PDP into 1 CDP, the other consolidates 3
PDPs into one CDP. Look at 10,20,30,40,50,60. One RRA has all 6 values,
the other RRA has average(10,20,30) equals 20, average(40,50,60) equals

minimum(10,20,30,40,50,60) equals 10. minimum(20,50) equals 20. Your test
would return 10 and 20.

It should work much better if you look at minimums of minimums:

The other RRA would contain min(10,20,30) equals 10, min(40,50,60) equals
40. Then min(10,20,30,40,50,60) and min(10,40) both return 10.

There can be two places where consolidation takes place. One is when data
is copied into RRAs, the other is when rrdtool graph needs data in a
resolution which is not available. If you take both into account, you
should be able to get predictable results.


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