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

Marc Powell marc at ena.com
Sat Sep 1 17:36:20 CEST 2007

> -----Original Message-----
> From: rrd-users-bounces at lists.oetiker.ch [mailto:rrd-users-
> bounces at lists.oetiker.ch] On Behalf Of Dan Gahlinger
> Sent: Saturday, September 01, 2007 9:31 AM
> To: alex at ergens.op.het.net; rrd-users at lists.oetiker.ch
> Subject: Re: [rrd-users] newbie rrd question - probably a very old

> 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
> weekly graph, that days spike has been changed to "30" and is plotted
> 30.
> this is very bad. it shouldn't happen.

Look at it this way. Assuming 5 minute samples, there are 288 data
points in 1 day. On a 300 pixel wide graph, you have enough space to
graph each single data point and then some. Now you ask rrdtool to graph
a weeks worth of data points in a 300 pixel wide graph. There are 2016
possible data points from which it must choose only 300 to display. How
should it do this? RRDTool uses the best fitting RRA and the
consolidation function you specify (likely AVERAGE). 

The fact that you only have two days of data doesn't mean RRDTool is
going to change the way it creates a weekly graph. You've told it to
graph a weeks worth of data so it'll do the best it can to show you a
weeks worth of data, NAN or not. You only had two days worth of data but
still asked rrdtool to show 7 days worth. It will, properly, consolidate
what you have to fit the two days and show 5 days worth of NAN data.


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