[rrd-users] How is the "now" value is calculated

[Steve Shipway]

Avast, ye swabs;  John Silver told I:
> Each graph has a "now" value display as part of that graph and I'm
wondering
> from where and how that value is calculated.  Our metrics are all defined
as
> gauges (because they com through various tooling like sflow or jmxtrans or
> gmetric. some support type, other do not).
> 
> The metric I'm collecting is an integer counter. But I'm sometimes seeing
the
> max (and last) value as NN.nn  where nn is not 00. Why would this (or the
> max value) never always be a whole number?

The short answer is Data Normalisation.  This is explained in Alex van den
Bogeardt's tutorial here: http://www.vandenbogaerdt.nl/rrdtool/process.php

The long answer is that RRDTool will adjust incoming values using a linear
approximation in time in order to make them fit into strict time samples.
So, if your RRD is set up with a 1-minute step, it expects all samples to
arrive precisely  on the 1minute boundary when the number of seconds is 0.
If the data arrive after this (such as at 12:00:15 instead of at 12:00:00)
the values are normalised to what they would have been expected to be at
this earlier time.  This results in the fractional values you are
experiencing.  It is important to realise that RRDTool treats all values as
Rates, and this normalisation does not affect the overall rate or total.

Steve


Steve Shipway
s.shipway at auckland.ac.nz

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