[rrd-users] Re: Weird #'s from basic rrdgraph

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Fri Mar 16 22:22:07 MET 2001

Clarence Hart wrote:
> Why do I get different values when I make my start and end times
> over aprox 13days?
> -> If I choose to graph 5 days the max value would be 140 pages.
> -> If I choose to graph 20 days the max value would change to ~70 
> pages.
> Why does this happen?

If you are working with the AVERAGE values, any peaks in the
data will be flattened out.  For instance:

A five-minute sample rate and also 5 minutes per row in the RRA.
t+00:05  value 120   (valid for interval t+00:00 to t+00:05)
t+00:10  value  80   (valid for interval t+00:05 to t+00:10)
t+00:15  value 100   (valid for interval t+00:10 to t+00:15)
t+00:20  value 100   (valid for interval t+00:15 to t+00:20)

If you fill two RRAs using this data, you'll get:

t+00:05  average 120 maximum 120  (valid for t+00:00 to t+00:05)
t+00:10  average  80 maximum  80  (valid for t+00:05 to t+00:10)
t+00:15  average 100 maximum 100  (valid for t+00:10 to t+00:15)
t+00:20  average 100 maximum 100  (valid for t+00:15 to t+00:20)

If this is consolidated into an RRA with 20 minutes per row, and
there is such an RRA for MAXIMUM values and one for AVERAGES, you
get two different numbers:

t+00:20  average 100 maximum 120  (valid for t+00:00 to t+00:20)

Note that the maximum seen is copied to the maximum RRA and the
average is calculated by averaging the averages.

So, you have four RRAs: 5 minutes per row and 20 minutes per row,
averages and maxima.

RRA0: 5-minute averages: 120,80,100,100
RRA1: 5-minute maxima:   120,80,100,100
RRA2: 20-minute averages: 100
RRA3: 20-minute maxima:   120

If you select the wrong RRA, you are selecting the maximum of the
averages and are thus getting 100, not 120.

I hope this makes it clear.


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