[mrtg-developers] Calculating 95th percentile

Westlake, Simon simon.westlake at twcable.com
Wed Oct 10 18:06:55 CEST 2007

Hello all,

I'm attempting to write a script to calculate 95th percentile usage
using RRD files generated using MRTG. I'm using PHP and PHP4RRDTool in
order to pull the information and calculate it and I'm running into an
issue which has maybe been hidden from me in the past as I use MRTG as a
collector for my RRD files.

A quick rundown of my method:

I have a loop that goes through and pulls 5 minute intervals of data
from the RRD file (by doing --start -10m --end -5m for example, and
moving backwards in steps of 5) for the last calendar month. It adds
this data to an array, sorts it from smallest to largest, strips the top
5% and calculates the average.

The script itself seems to work fine, but the data I get returned seems

After reading the RRD documentation, it appears that I should be able to
pull the information I want by using the following command:

$opts = array ( "AVERAGE", "--end", "-$timeframe", "--start",

Where the $opts array is an array of the options to send. This works and
gives me a value, however, the values seem to repeat identically. Here
is a (somewhat brief) example of the data returned over a series of 5
minute queries:

<34784619.7841 repeats many times>

This seems wrong to me - even as an average, I would not be seeing
identical values over separate 5 minute intervals.

I have tried various changes to see if I receive different information,
and I do not.

Am I missing something blindly obvious here? Shouldn't different 5
minute intervals return different information?

Simon Westlake
Time Warner Cable Business Class
Network Engineer
Ph: 414.908.4791 | Cell: 414.688.7956
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