[rrd-users] what is Upper Lower Trend and 95th

[Steve Shipway]

> Hi all,I have ntop with rrd tool installed and is working ok. I make
> some
> graphics of my network but my boss want to know what values on graph.
> present (upper, lower, trend and 95th percentile)

These are all statistical analysis functions of the data.  To fully understand them, you'll need a fair amount of mathematical knowledge, but in brief...

Trend is the 30min average; in other words, a smoothed-out approximation of the current data, showing the current trend using a 30min window.  This removes the more short-term spikes to give you a better indication of the way the data are trending.

95th Percentile is the 905th Percentile calculation for the data set currently being viewed.  This is a statistical calculation that is generally accepted as a good indicator of the usual required peak capacity.  It is calculated by taking all data samples, sorting them, and finding the value which is greater than or equal to 95% of the available data.

Upper and Lower are the confidence bounds for Holt-Winters statistical analysis.  They show the range of value between which the current value is expected to lie, based on past experience.  How accurate these are depends on what settings were given to the analysis when initially set up, particularly with regard to the expected periodicity of the data.  In general, if the observed value is outside of this range, it should be considered 'unusual' for this metric.

Hope this helps.  Much more information about 95th Percentile, Holt-Winters analysis, and trending is available in the RRDTool documentation at rrdtool.org...

Steve
  

Steve Shipway
ITS Unix Services Design Lead
University of Auckland, New Zealand
Floor 1, 58 Symonds Street, Auckland
Phone: +64 (0)9 3737599 ext 86487
DDI: +64 (0)9 924 6487
Mobile: +64 (0)21 753 189
Email: s.shipway at auckland.ac.nz
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