[rrd-users] How to RRDgraph differences over time (derivates)

Søren Schrøder sch at cybercity.dk
Mon Aug 29 12:00:38 MEST 2005

Greetings rrd-users

I have been using RRDtool for years, for mutiple tasks, and every time I
invented a new need for wierd graphing in RRDtool, it seemed there
always was a way to do it. But this time the man pages doesn't give me a
hint on how to do it, so I try the list.


X.RRD countaining one dataset, N (absolute), logged every 5 minutes.

It could be number of active users on a system.

I would like to do a rrd_graph where i plot the DIFFERENCE between last
logged data, and the data logged, say 10 minutes ago. This derivate
could be used to detect unnormal changes in population (if I loose 10000
users in 10 minutes on a populatin of 200000 users, it probably because
somthing went wrong (or the important soccergame in TV started)

I imagine something like (in pseudo code): Nmark = ( N(now) -
N(now-10minutes) ) / N(now) * 100

and in the above example: Nmark=(190000-200000)/190000*100 = -5.26%

At the moment I do this by keeping a state of the "10 min ago" value on
the data-collector, and then pass it to the rrd as a second dataset, but
this is in my book redundant data, as it already exists in the rrd, and
it not very flexible either, if I would like to look at 5 minutes and 30
minutes changes too.

Any ideas on howto ?

Kind regards

Søren Schrøder

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