[rrd-users] Question with 1-hour samples
linux at thehobsons.co.uk
Thu Jan 20 16:25:08 CET 2011
>I'm beginning to suspect that my problem is not understanding and
>therefore using the CF functions correctly on my data, for MAXIMUM
>versus AVERAGE. That is why my 5-minute averages look correct to me
>(and what I'm intending to do), while my 1-hour averages are not what
>I'm trying to achieve with my graph.
OK, in short, lets assume you keep stuffing values in at least as
often as your step size.
Firstly, RRD will normalise your data to get a SINGLE value for each
STEP period. So, for example and assuming 5 minute (300s) step size,
you will get a SINGLE value for the period 03:05 to 03:10. This is
called a Primary Data Point (PDP). It does not matter if you feed in
a new value every second - you only get ONE PDP for each STEP period.
Min, Max, Average have no bearing at all on this as there is only
ever a single value for the step.
Next, every so often you will complete the number of steps required
to fill a consolidated data point. If, for example, you consolidate
12 PDPs into a Consolidated Data Point (CDP) - this will happen on
the hour since all time calculations are referenced to Unix epoch
(Midnight, 1st Jan 1970 UTC).
So, you keep stuffing in updates, and come 04:00 RRD will take the 12
PDPs covering 03:00 to 04:00 and consolidate them.
For an AVERAGE consolidation, it will take the average. For a
MAXMIMUM consolidation it will take the maximum, and for a MINIMUM
consolidation it takes the minimum. Note that in all cases, these are
not calculated from the values you put in, but from the normalised
values stored in the PDPs.
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