[sdnog] cacti and old data values
Nishal Goburdhan
nishal at controlfreak.co.za
Sun May 8 13:00:04 SAST 2016
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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