[rrd-users] Unexpected decimal values in RRA

Steve Shipway s.shipway at auckland.ac.nz
Mon Nov 3 05:11:07 CET 2008

> When I fetch the values I see this:
> 1225359000: 3.8814344466e+03 -7.3078100609e-05
> 1225360800: 3.8815660814e+03 7.3156179499e-05
> Why do I get the decimals from time to time? It seems every time the
> polled value changes I get a "soft" change over three samples.

Data time window normalisation.

The RRD has 'buckets' at (eg) 5-min intervals.  If your sample is logged EXACTLY on the boundary of the bucket, that's what you see.  Otherwise, it draws a line from the last value to the new sample, and logs where it passes the boundary -- ie, normalises to the exact time point.  This is in the RRDtool docco but it's a bit weird to understand - you need to remember that RRD was originally created for graphing network traffic data rates and then it makes sense.


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