[rrd-developers] Re: RRD eating values

[David M. Grimes]

On Fri, Oct 08, 2004 at 10:37:55AM -0400, Sunil Modi (IT) wrote:
> I generated some backlogged data from January of 2004 into an RRD.  I created the RRD one month at a time (tedious but RRD had problems taking all the data in one large update file).  I had run a test case of the first month to see how the data would look and it looked just fine when compared to excel graphs.  
> When I finished making the RRD and putting in all 9 months of data, I generated some graphs, and saw that the graphs are different and the peak points are different.  The single Month Graph had a peak of 100, and the 9 Month graph had a peak of 60.  What happened to the peak of 100?  Should I make separate RRD's for each month to be more accurate?

The effect you're seeing is the result of having too much data density to
fit on the graph you are generating.  For a 9 month graph at default
resolution (400x100) you've got about 16 hours worth of data that get
"rolled up" to form a pixel on the graph.  If you're using the AVERAGE CF
for the archive, you'll be averageing 16 hours worth of data, which is
naturally going to remove the "peak".

If you're using the CVS snapshot (development branch) after about
09-27-2004, you can use this new option on the DEF: when you draw the graph:

DEF:x=rrd:ds:step=<step override>:reduce=<CF>

And the step override should be the granularity of your "highest density"
RRA, and the CF for reduce would be MAX to achieve what you're trying to
see.

Hope that helps,

 --Dave Grimes

> 
> Thanks,
> 
> Sunil Modi
> IT-Server Administration
> (734) 477-1570
> smodi at bordersgroupinc.com
> 
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