[sdnog] cacti and old data values

Samir S. Omer samir.saif at sudren.edu.sd
Mon May 9 04:55:48 SAST 2016

----- Original Message -----
> From: "Nishal Goburdhan" <nishal at controlfreak.co.za>
> To: "Samir S. Omer" <samir.saif at sudren.edu.sd>
> Cc: sdnog at sdnog.sd
> Sent: Sunday, May 8, 2016 3:00:04 PM
> Subject: Re: [sdnog] cacti and old data values
> On 8 May 2016, at 9:44, Samir S. Omer wrote:
> > Hi all
> >
> > can someone explain to me how does cacti/rrds work exactly in term of
> > storing values ?
> > I noticed that the values get inaccurate when time is defined for old
> > times
> >
> > for example when I show the graph for last two days it shows a spike
> > in traffic. However, if I set the time for the last week I see the
> > graph is gone and graph is normal.
> cacti uses rrdtool as a base;  so, yes, after a while, the law of
> averages means that you’ll lose granular reporting.
> are you asking about the algorithms used for calculating these
> “averages”?
> i’ve never really dug that deep into it, but you can probably find
> more information here:  http://oss.oetiker.ch/rrdtool/

thanks, yes this is what I was searching for 

> you might also find that your cacti installation has additional plugins
> to remove traffic spikes.
> from one of doc pages:
> http://oss.oetiker.ch/rrdtool/pub/contrib/spikekill-1.1-1.txt
> “The spike killing methodologies supported in this utility are as
> follows:
>    - Remove via Standard Deviation
>    - Remove via Variance from Average
> The Standard Deviation method first calculates the average of all Data
> Sources and
> RRA's and determines the Standard Deviation of each combination.  It
> will then
> remove any sample over X Standard Deviations from the Average.  The
> utility has the
> option of replacing the spike by the average, or by a NaN at your
> discretion.
> The Variance Method first calculates the average of all Data Sources and
> RRA's.
> Before doing that it removes the top X and bottom X samples from each
> sample set
> treating them as outliers.  Then, based upon the revised average will
> remove
> all spikes Y percent above or below the revised averages.  Again, like
> the
> Standard Deviation method, you have the option of replacing the spike
> with the
> revised average or a NaN.”

well, in my case I was searching for spikes I don't want to remove them, but
I've used this plugin with other graphs and it mostly delivers what is needed

> hth,
> —n.

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