[rrd-users] Displaying "difference" or "Change" over the displayed period
Alex van den Bogaerdt
alex at vandenbogaerdt.nl
Thu Mar 8 03:19:51 CET 2012
> I have been using RRD for quite a while now, and I would like to
> replicate something that my old weather station gave.
> That is, how much rain has fallen over the previous 24Hrs (though
> the weather station previous 24Hr reset at midnight) I'm sure RRD would
be able to do the full 24Hr, or full time scale -24H or -48H...
RRDtool is based on UTC time internally. Look through the archives if you
are not aware of this.
> But I cannot work out how to get it to display (MAX-MIN) to =
> DEF:rain-max=rain.rrd:R24h:MAX \
> DEF:rain-min=rain.rrd:R24h:MIN \
> VDEF:MaxRn=rain-max,MAXIMUM \
> VDEF:MinRn=rain-min,MINIMUM \
> CDEF:R24Hr=rain-max,rain-min,- \ <- This doesn't work
I think it does work, but does not do what you want it to do?
Maybe you also tried:
This CDEF is not supposed to work, it would be nice if this VDEF would
work, but nobody programmed this yet.
Try this hack, you may need to tweak it a bit:
Unless one of the variables involved is an unknown or infinite, this
should result in one straight line at (MaxRn-MinRn).
I start with one of the available DEFs (a CDEF would also do).
Then I add MaxRn minus MinRn.
Then I remove the DEF again.
Why add and remove the DEF? Because a CDEF needs to now how many buckets
and how much time per bucket. This property comes from the DEF and stays,
even after removing the values from the DEF.
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