[rrd-users] trying to use PREV in a CDEF

[Clarence Gardner]

I have a GAUGE data source that I need to graph two ways: one, just the
normal data, and two, the difference between successive samples.
E.g., for data 1000,1010,1050 I want abscissas of 1000,1010,1050 on the
first graph and 10,40 on the second.  Obviously a job for PREV, I thought.

I tried
  CDEF:delta=n,PREV,-
but got nothing on the graph.  I'm thinking this is because the UNKNOWN
value that it got the first time propagated to each value in turn.  So
then I tried
  CDEF:delta=n,PREV,UN,n,PREV,IF,-
to get rid of the UNK at the beginning.  This works once, and returns, e.g.,
10 as the difference between 1000 and 1010.  Unfortunately, it also makes 10
the value at that ordinate, thus the next cycle calculates 1050 - 10 rather
than 1050 - 1010.  This makes for a graph that oscillates wildly.

Am I missing something here about how to use PREV?  Thanks for any help.

-- 
Clarence Gardner
Software Engineer
NetLojix Communications
clarence at netlojix.com

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