[rrd-users] Consolidation Happening even through I don't want it to!!

[Tobias Oetiker]

Hi Donald,

the indenting of this message is realy confusing, so I may be
responding to the wrong thing here ...

Today O'Neil, Donald A. wrote:

[...]

>
> Excel automatically graphs whatever size you want the graph to be, and
> anti-aliases/scales things appropriately. I don't know how it does it,
> just that if I graph the data in Excel it looks MUCH different than in
> RRDtool...
>
> The main point is that the MAX and MIN values in the raw data do NOT
> match the calculated MAX and MIN values on the graph. It appears as
> though RRDTool calc's these max/mins from its own 'squished' data.
>
> I think the issue here is how RRDtool interprets so many data points to
> create a graph that is only 500 pixels wide. I would think rather than
> averaging the data it should take the absolute value of the max as the
> point it plots, rather than taking an average of a number of points and
> plotting it. I would bet this is how Excel does it.

when rrdtool has to 'squish' 10 points into 1 it uses whatever
consolidation function has been active for generating the 10 points
(see the DEF line) to come to the 1 point ... in newer rrd version
(1.2 and up) you can override this default with the reduce option
in the DEF line.

Lets say you have 3 Points:

  10, 4, 1

If the 3 points are from an AVERAGE DEF rrdtool would draw the
pixle at

  10+5+1 / 3 = 5

If the 3 points are from a MAX DEF then the pixel would go to

  10

If they were from MIN

  1

Using the reduce option you can even have RRDtool show you MIN, MAX
and AVERAGE from an AVERAGE DEF.

I rest my case ...

cheers
tobi



-- 
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