[rrd-users] newbie rrd question - probably a very old issue

[Dan Gahlinger]

I think you're missing the point.

true, the graph only has so much space, but I call "BS" on your overall 
explanation for 3 reasons.
#1 we don't have that MANY points to plot - so there's more than enough 
space on the graph to show everything
#2 - even so it shouldn't be changing the actual points it does show. a peak 
of 90 should be a peak of 90 regardless, it shouldn't shift it to a peak of 
30, no matter what the numbers are. - if you can see the point on the graph, 
it should be the same value.
#3 - using "max" instead of, or in addition to "avg" for RRA is a bogus 
answer. if we use "max" instead of "Avg" we lose the "min", and the "avg" is 
skewed. if we use it in addition, we have to define, "min", "avg" and "max" 
- three RRA's for one data set, which kills the load on the server. imagine 
1000 service checks having to write 3x the data instead of one. this isn't 
the answer. the answer is to find out why RRD/RRA is doing what its doing 
and make it stop.

in our case, we only had 2 days worth of data, actually less than 1000 bits 
of data.
so on the weekly graph there are enough points.

on the graph we can clearly see the "90" spike on the daily graph, but the 
weekly graph, that days spike has been changed to "30" and is plotted as 30. 
this is very bad. it shouldn't happen.

but again, it's not the graph function that's doing it.
with the command line query, it produces the same results.

the graph is graphing it that way because RRD is giving it the data that 
way.
RRD command line query for the week retrieves the max value as "30"
even though the raw data shows the "90" is there.

this is definitely broken. and badly. I'm not sure what's wrong with it.
again, it could be our fault in how the RRA is setup, but I don't see how.

I'll post that as soon as I can.

>From: Alex van den Bogaerdt <alex at ergens.op.het.net>
>To: rrd-users at lists.oetiker.ch
>Subject: Re: [rrd-users] newbie rrd question - probably a very old issue
>Date: Sat, 1 Sep 2007 13:10:30 +0200
>
>On Fri, Aug 31, 2007 at 10:06:24PM -0400, Dan Gahlinger wrote:
>
> > you may be correct that we are asking it to consolidate, and we don't 
>want
> > to. we simply don't know.
> > it just seems, totally wrong.
>
>If you have 400 pixels on the graph, and you have 4000 averages to display,
>it won't fit. Somehow you will have to combine 10 intervals into one.
>If you don't, then rrdtool will.
>
> > what's the point of defining so many data points if its going to
> > consolidate it anyhow?
>
>With many datarows you can look back in time at the highest possible
>resolution. You won't be using all of the data, just a portion of it.
>
>Consolidated data can be used to quickly report on a longer interval,
>without having to wait for rrdtool to do "consolidation" on the fly.
>No need to reduce 4000 rows into 400, if you already have the data
>available in a more appropriate resolution.
>
>They both serve a purpose.
>
>
> > we have learned it is NOT the graphing function doing it, though.
>
>then it has to be another available RRA.
>
> > if we look at the raw data it is correct, but if we query the data, we 
>get
> > the consolidated results.
> > it shouldn't be consolidating.
>
>Then you shouldn't have defined that RRA.
>
>More likely you should have created another RRA, or if you did, you should
>be using it.  If you want to see maxima, then you should use the MAX RRA,
>not the AVERAGE RRA.
>(capitals not for shouting, but because that's how it's written!)
>
>--
>Alex van den Bogaerdt
>http://www.vandenbogaerdt.nl/rrdtool/
>
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