[rrd-users] Can't understand rrd way of storing data

[Steve Shipway]

>I've just discovered rrd and it seems to be the perfect
>tool for collecting statistical data. Anyway I don't really
>get how rrd does to collect data.
>I just want to get exact values at exact times.

You can't (unless you set the storing time to be exactly a multiple of the step size)

RRDTool is designed to normalise data into predetermined 'bins' defined according to the RRD step size.  If you store data for a time anywhere in the bin interval, the data will be adjusted relative to the previous sample value and time and the time of the bin start.

This is great for continuous, regular data polling and graphing of, for example, network traffic - what RRDTool was designed for.  It is not suitable for random data samples, which seems to be what you're using it for.  You'll also notice that you can't store data backwards in time from the last stored sample and that if you leave a too long gap between samples (eg, twice the step size) you'll start to get 'unknown' stored instead on the  lower-granularity RRAs.

RRDtool is great for the purpose for which it was designed.  It is not, however, one-size-fits-all.  Your requirement may be for a different statistical engine

Steve