[rrd-users] RRA:AVERAGE - How does RRDtool know which RRA to use?

Simon Hobson linux at thehobsons.co.uk
Wed Aug 18 22:00:09 CEST 2010

tinsfci wrote:

>Let´s say I plot a graph from "now" to -1month, does RRDtool "automatically"
>use "no 3", or do i specify it somewhere?
>"no 1": 600 samples of 5 minutes  (2 days and 2 hours)
>"no 2": 700 samples of 30 minutes (2 days and 2 hours, plus 12.5 days)
>"no 3": 775 samples of 2 hours    (above + 50 days)
>"no 4": 797 samples of 1 day      (above + 732 days, rounded up to 797)

Best to be more accurate in your start and end times.

For starters, don't end "now", end "now - (now % 
interval)" where interval is the period for the 
consolidation you want to use. Taking your 
example, you would only ever use 00:00, 02:00, 
04:00, and so on for your monthly graph.

For start time, again be precise. I will select 
"end time - (interval * number of pixels)" so 
that the start time is exactly on a period 
boundary and each sample fits to one pixel in the 

For the consolidations you've used there it won't 
happen, but there are some combinations that can 
cause you to get low resolution data. rrd tool 
will use whichever consolidation provides the 
most data to fill your graph - so if you manage 
to have odd periods you may find your one day 
graph drawn with data consolidated at one day !

As a simple example, suppose you have 
consolidations at 10 minute and 25 minute 
resolutions. For some values of "now", there will 
be a more recent value in the 25 minute 
consolidation than the 10 minute one. Eg, if they 
happen to coincide at midnight, then between 
00:25 and 00:30 this would be the case.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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