[rrd-users] Best way to track absolute data.

Simon Hobson linux at thehobsons.co.uk
Mon Feb 1 21:14:22 CET 2010

Shem Valentine wrote:

>I have an ABSOLUTE counter that I'm trying to monitor (resets after each
>read).  This counter is reset every 30 minutes.
>I'm trying to make my rrd files as minimal as possible, as I will be
>tracking thousands of these monitors.  So I would like to have a 30-minute
>archive for 3-4 months, and a 24-hour archive for 5 years.
>It is important to me that the RRD contains accurate data totals, and not

RRD Tool does not store values. EVERYTHING you put in it is 
normalised and converted to a rate - Alex has an excellent tutorial 
on that at http://www.vandenbogaerdt.nl/rrdtool/

What you can do, with sufficient care, is to enter your data such 
that normalisation becomes a null operation and then you can compute 
the total by multiplying rate by time. This will be subject to 
rounding errors.

>   The 30-minute archive is simple enough, an ABSOLUTE DS, with a
>LAST RRA.  However, I'm struggling to get the 24-hour archive.
>I could do it similar to above, a 24-hour RRA using LAST, and have my
>script that gathers the PDPs, total them, and update an ABSOLUTE DS.

LAST gives you the last computer PDP (primary data point). Thus if 
your step was 30 minutes, it would give you the value for the 23:30 
to midnight timeslot and completely ignore midnight to 23:30 (both 
times UTC). What you actually want is AVERAGE - and then multiply the 
rate returned by 86400 to get a total count for the day.

Simon Hobson

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