[rrd-users] A reason there isn't a CF SUM function?

Simon Hobson linux at thehobsons.co.uk
Wed Aug 22 09:27:06 CEST 2012

Tobias Oetiker wrote:

>you can always 'convert' the unknowns to zero first using an
>appropriate CDEF

Does that work when the data has been consolidated ?
Eg, suppose you have 10, 10, 10, ... unknown, 10, and consolidate 10 
values (9x 10, 1x unknown) into one CDP. Do you get 10 (average of 
the 9 known values) or 9 (average of the known values and assuming 0 
for the missing one) ?

The problem is that by the time you come to graph the data, you only 
have the consolidated average to work with, and 10 would give the 
wrong answer - as could 9, it depends on your requirements.
If the missing data is because we've had the power off, and so had a 
step period where nothing happens, we'd expect to have done/moved 90 
<somethings> during the consolidation period, not 100.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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