[sdnog] cacti and old data values
Samir S. Omer
samir.saif at sudren.edu.sd
Tue May 10 09:06:30 SAST 2016
it's clear now
----- Original Message -----
> From: "Frank Habicht" <geier at geier.ne.tz>
> To: sdnog at sdnog.sd
> Sent: Monday, May 9, 2016 11:43:19 AM
> Subject: Re: [sdnog] cacti and old data values
> RRD (round-robin database) are meant to not increase the size over time.
> We know new data is getting into the database.
> So it will have to remove some (details about) older data.
> One can configure the parameters.
> We can take samples every 5 minutes, keep 2 days worth of these samples.
> 2days * 24h * 12 samples/h records in there.
> we want to keep older data, up to 2 weeks, but only regarding each
> 30-minute interval.
> 2weeks * 7 days * 24h * 2 samples/h records in there.
> we want to also keep older data, up to (your choice, btw) 3 months worth
> of these samples, but with a granularity of 2-hour intervals.
> 3 months * ~30days * 12 samples/day records in there.
> You see that the size of the database will not increase.
> but every 30 minutes we have to "forget" / remove all the 6 samples that
> were in the 30-minute interval 2 days ago.
> How do we make one value out of the 6 ?
> rrdtool defines some "consolidation functions" and you can choose among
> them. You can use the average, the minimum, the maximum and other more
> complicated ones, I believe.
> Most often used is the average. (that removes your spike)
> For network operations the maximum should also be interesting. (that
> would keep your spike)
> Maybe `man rrdtool` is an even better source. Search for 'Round Robin
> `man rrddump` gives info about extracting all these details out of an
> RRD file.
> On 5/8/2016 2:00 PM, Nishal Goburdhan wrote:
> > 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/
> > 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.”
> > hth,
> > —n.
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