[rrd-users] the usage of CF

dophine dophinehk at hotmail.com
Thu Sep 24 08:44:40 CEST 2009

Alex van den Bogaerdt-5 wrote:
>>>> I have question using CF MAX and LAST. I have 80000 data point for each
>>>> second.
>>>> And I plot a chart with the step size 1. However, I found the chart 
>>>> using
>>>> MAX CF and LAST are different.
>>>> Does anyone know the reason?
>>> Step size 1 means 1 second.  You claim to have 80,000 datapoints per
>>> second,
>>> I'm only going to show 5 per second. This is what happens when 5
>>> datapoints
>>> are consolidated into one:
>>> LAST(10,20,30,20,10)==10
>>> MAX(10,20,30,20,10)==30
>> I am sorry. 1 data point is for one second. That means there are total 
>> 80000
>> seconds data.
> And how many of those are you showing in your chart, and how wide is that 
> chart?
> Consolidation may still happen, at graph time.
>> As the step size is 1, I expect the MAX and LAST should be the same.
> No. See my previous remark. Or it is normalization which kicks in. This 
> will, with step==1, only happen if you do not use timestamps to update,
> like 
> so: rrdtool update your.rrd N:12345
> (I am assuming that not only step==1 but also steps==1, 1 step per RRA
> row)
> In such a case updates may not be exactly on the second, it may for
> instance 
> be 07:30:00.05
> See my site, http://www.vandenbogaerdt.nl/rrdtool/ , item Rates,
> normalizing 
> and consolidating
> First make sure you understand what is written there.  If this still does 
> not help, write a small script which:
> 1: creates a small and simple database
> 2: updates a couple of times
> 3: creates a small and simple graph.
> This script should show the behaviour which you are experiencing, there's 
> really no need for many DS's, a gazillion updates and a fancy graph.
> Alex
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> rrd-users at lists.oetiker.ch
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I update each data point at one second. After reading your site, if I want
to show all the 80000 data points, I need set the width of graph to 80000
too, right?
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