95th Percentile - how is it calculated?

[mikepont]

Hi,

I have the 95th percentile feature up and running and was wondering how the value is calculated over the various timeframes - daily, weekly, monthly & yearly.

For billing I know that UUNET takes 5 minute average utilisations of a customer circuit across a whole month and then calculates the 95th percentile of the 8640 or so samples (30days x 24hours x 12samples/hr) = 8640 samples.

Is the Cacti 95th percentile value calculated for a month based on 2hr average values? I guess this would yield a different result to using 5min average values so comparing an ISP's calculated value with Cacti's for a month would give different results?

[raX]

You are pretty much on track. Cacti calculates the 95th percentile with whatever resolution RRDTool is storing for the given time interval. By default you will get a five minute average for daily, thirty minute average for weekly, etc.

If you plan on using it for billing and want a monthly five minute average, you can modify your monthly RRA:

Steps: 1
Rows: 8928 (31 days)

If you make this change you would have to delete all of your RRD files and let Cacti re-create them for the change to take affect.

-Ian

[cwlab-admin]

ive done this change and the rrd files were regenerated perfectly. but the monthly graph only shows the past seven days. if i read the source correctly it starts the time scale at -14.4 days and that doesnt even come across correctly

http://traffic.cwlab.net/graph.php?loca ... ource=true

[cwlab-admin]

there is a hardcoded value that affects the start time from the number of samples.

x = number of samples
y = time in seconds

144x=y

need to store 4200 5 min samples for one week and 18000 samples for 30 days