[rrd-users] Re: Graph data is affected by the size of PNG image

Dan Cech dcech at phpwerx.net
Thu Jan 4 20:17:11 MET 2007

Alejandro Galue wrote:
> Hello Dan,
> Thanks for your quick response. I really appretiate that.
> I included an attach with two files inside:
> the RRD called  379.framerelay1.rrd
> a perl script called graph.pl
> This file generate the graphs for the tests.
> The small graph use (500x150) and the big graph use (800x300)

After looking through your attachment, I see that you are indeed using a
DS defined with AVERAGE, then attempting reduce=MAX.  This is definitely
part of your problem, and will never give you consistent results.

You should be consolidating the data using an RRA for each of the types
of data you wish to extract, so in your case you'd want and RRA for
MIN,MAX,AVERAGE and LAST for each DS at each resolution.  Then you can
use the correctly aggregated data to get accurate figures out.

Right now, your data is first averaged when going into the RRA, then
reduced with MAX, then aggregated in your graph.  So you end up with the
maximum of the maximum of the average, the average of the maximum of the
average, etc.

> I used "step=300:reduce=MAX" and "step=300" alone with no success, but
> the graphs is a little bit better than with or widthout "reduce=MAX".

By fixing the step size, you're reducing the change between the 2
graphs, by forcing them to use the same RRA, but your figures will still
be incorrect.

> reduce=MAX is used for pixel reduction technique and is not related to
> consolidation function used to fetch data; am I right ?
> Is this a Bug ?

See above, your data is not being consolidated in a way that would allow
rrdtool to answer the question you are asking.


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