[rrd-users] How Does RRDTool Produce Chart with MANY Data Points

[Tobias Oetiker]

Tomorrow Mark Easton wrote:

> Hi Tobi,
>
> I think you have missed my point.  Please ignore the ave, max, min stuff I
> spouted on about (I edited my post online).
>
> The RRA I am using in my graph has no consolidation. It presents for one
> month, about 40,000 points (60 second readings from my sensor - my step is
> 60). ie
>
> RRA:AVERAGE:0.5:1:40320   (I assume that AVERAGE, MAX, MIN of 1 value will
> all be the same??)
>
> My question is that when I ask RRDTool to produce a graph for this does it
> just plot all 40320 points or does it do some optimisations based on the
> size of the graphic being produced? For 12 months, it would need to plot
> 524,000 odd points.

Mark,

I think I have understood you perfectly (you are only about to
1000thst person to ask this question). But I guess I have still not
reached the point where I am able to explain this to people so that
they understand in the first round ...

If you plit 10'000 points to a chart 1000 pixel wide, then rrdtool does NOT plot
every point, but it takes groups of 10 points and figures out where
to draw the pixel in the graph. The method it uses for this process
it the same that was asigned to the RRA unless you add a reduce
optino to the DEF line when pulling the data in.

Since you did not add a reduce option you see the average of 10
point plotted.

For performance reasons it makes sense to have additional RRAs
roughly matching the graph resolutions people will be looking at.

hope this helps
tobi


if you like, I can repeat for you what I wrote yesterday.
>
> Cheers,
> Mark
>
> > -----Original Message-----
> > From: Tobias Oetiker [mailto:tobi at oetiker.ch]
> > Sent: Tuesday, November 18, 2008 7:53 PM
> > To: Robert Easton
> > Cc: rrd-users at lists.oetiker.ch
> > Subject: Re: [rrd-users] How Does RRDTool Produce Chart with
> > MANY Data Points
> >
> > Hi Robert,
> >
> > Yesterday Robert Easton wrote:
> >
> > >
> > > Hi,
> > >
> > > If I ask RRDTool to plot a chart with 10,000 data points it does so
> > > very fast. What is RRDTool doing in the code? It appears to
> > me that it
> > > is averaging or smoothing the data. Is that right?
> > >
> > > I was actually plotting readings from a temperature sensor
> > taken every
> > > minute for one week. So that is 60 x 24 x 7 = 10080 data
> > points. In a
> > > patrticular day the temperature might have a min of 8 and a
> > max of 24,
> > > but the resulting chart did not manage to show that min and
> > that max
> > > in a particular day.
> > >
> > > So what does it do?
> >
> > rrdtool uses the same consolidation function that was defined
> > in the RRA to further consolidate the data so that it matches
> > the resoltion of the graph ... if you want to see min and
> > max, setup additional rras with MIN and MAX consolidation functions
> >
> > OR
> >
> > use the reduce option in the DEF line to override the reduce
> > function with MIN or MAX respectively.
> >
> > cheers
> > tobi
> >
> > >
> > > Thanks,
> > > Mark
> > >
> >
> > --
> > 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 No virus found in this incoming message.
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> > Date: 11/17/2008 5:24 PM
> >
>
>

-- 
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