[rrd-users] Dynamic Steps

Oliver Loch grimeton at gmx.net
Fri Apr 19 18:43:09 CEST 2013


> But in the case of Wh, I recommend to multiply 'ticks' by 3600 at input 
> time, because each tick is worth 3600 Joules, and this results in a clean 
> database storing Joules per second aka Watt. Although this could also have 
> been done in a CDEF, it just does not "feel right".

How could that be done with an CDEF? Let's assume the W/h example. As far as I understand it and based on the information in your e-mail, the CDEF language only works with CDPs. So if there is a CDP every second, and the CDP - steps can't be below one second, multiple ticks per second would be used to calculate the average of the CDP-step and stored in that particular CDP.

So imho trying to solve this via CDEF wouldn't work as we already lost the exact data that we'd need to calculate it. Only thing I could think of would be to add more steps per second (which isn't possible) or trying to use more CDP per hour. Like 7200 which would give me "CDP Steps" at the range of 0.5s. But as this isn't possible one idea could be to just "jump" multiple steps per second and use that information later when creating the graph. E.g.: Four "steps" per second would give me 14400 steps per hour. So every second stuff is written to a CDP it just forwards four steps instead of one. and only fills as much steps as there were ticks. So four CDPs per second, three ticks that second, it moves forward 4 CDPs and then fills the last three. 

But I also have another question that isn't really clear to me:

Let's assume again that I'm monitoring the power meter.

The database has one source, which is the "tick"-counter that has 3600 steps and uses the ABSOLUTE counter. Now I add multiple RRAs. First one is for an whole hour with an per minute average. So I store 60 CDPs that are created with 60 CDPs from the datasource. Now I'm also creating an RRA that holds the average per hour and requires ??? CDPs to populate one CDP. The question is - if defining multiple RRAs, are they all referencing the main data source or are RRAs that store data over a long time, less exact, referencing to the "smaller" RRA?

So. what happens if I have 3600 steps per hour and the only RRA does the average of two hours? Is rrdtool smart enough to handle that?



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