[collectd] generating composite metrics

[Gerardo Herzig]

Hi all. Im wondering if it is possible to generate a metric as a calculation between others.

Example I: Know the rate between the "used memory" and the "total memory".
Example II: Know the rate between "In use" apache threads against "total" apache threads.

I know how to make a graph as this sort of calculations, but i also need to generate an alert when, say, 75% of the apache's threads are in use, so the graphical solutions does not cover all my needs.

So i guess the correct solution is to generate my own .rrd files according to existing .rrd files. I just dont have any idea how.

If some of you can give me a topic, or man page to look at, it would be great.

Thank you all.
Gerardo