[rrd-users] comparing to past samples

[Alison L Greenwald]

I am collecting data from an SNMP counter into an rrdtool database.
I'd like to be able to plot effectively the average number of
connections over time.  So, for example, if I get (at 5 minute
intervals):
5, 10, 12, 20, 23
I want to plot the difference from the previous, value, i.e.:
<anything>, 5, 2, 8, 3 

If I store this data in a COUNTER datatype, all I get is the
percentage change over the previous value, which is not what I want.
I thought I could do this using PREV, but I can't see how...

DEF:a=$FILENAME:total:MAX
CDEF:b=a,PREV,UN,a,PREV,IF,-

woudl give me:
0, 10, 2, 18, 5
which is not really what I want.

I can't do
CDEF:b=PREV,UN,a,PREV,IF
CDEF:c=a,b,-
CDEF:b=a

because I can't reassign b.

Is there anything I can do to get this to work?

Thanks in advance,
Alison 

Andrew Systems Group
Carnegie Mellon University
 


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