[rrd-users] unexpected behavior of GPRINT and DEFS with start= args

[Tobias Oetiker]

Hi Martin,

Yesterday Martin Sperl wrote:

> OK, the solution I found was to Use 2 DEFS:
> * one without the start parameter (say avg)
> * one with the start parameter (say avgext)
>
> and then calculate the prediction using avgext
> and calculating the difference between the (extended) PREDICTION and the unextended value.
>
> The subtraction will become NAN for those values that only exist in "avgext" and thus are not included in the MIN/AVERAGE/MAX calculation of GPRINT.
> So with this I get the expected value of 8.6 and not the value 0.28.
>
> Still the question is: is this expected behavior for GPRINT?

it is the expected behaviour ... you could use this to show
yesterdays average for example ...


cheers
tobi

>
> Ciao,
> 	Martin
>
>
> On 30.06.2011, at 19:54, Martin Sperl wrote:
>
> > Hi!
> >
> > I have a unexpected behavior with a graph that is created similar to this (simplified):
> >  ...
> >  --start=1309282387 \
> >  --end=1309285987 \
> >  'DEF:avg=test.rrd:value:AVERAGE:start=1308677587' \
> >  'LINE1:avg:average' \
> >  'GPRINT:avg:MIN:(%9.2lf' \
> >  'GPRINT:avg:AVERAGE:/%9.2lf' \
> >  'GPRINT:avg:MAX:/%9.2lf' \
> >  'GPRINT:avg:LAST:/%9.2lf)\l'
> >
> > I was assuming that the GPRINT would only be calculating the min/max/average/last for the area really graphed (3600 seconds).
> > But it seems it is calculating it for the whole 7 days and 1 hour (=608400 seconds) that got loaded in DEF.
> >
> > Is this "expected" and/or "intended" behavior?
> >
> > If it is, is there an easy means to get what I really need - just the values for what I graph.
> >
> > You may ask why I use "start=" in the DEF in the first place.
> >
> > The explanation is that I essentially need it for predictions using the PREDICT CDEF function.
> > And it came as a surprise when I was trying to estimate the AVERAGE of a graph zoomed in on an "outage", which showed the difference between the average and the prediction (using "CDEF:delta=avg,604800,604800,518400,432000,345600,259200,172800,86400,8,1800,avg,PREDICT,-")
> > I was expecting an average of -8 as the average but got 0.28, which would be realistic for the whole 7 days.
> >
> > Thanks,
> > 		Martin
> >
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>
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-- 
Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
http://it.oetiker.ch tobi at oetiker.ch ++41 62 775 9902 / sb: -9900