[rrd-users] multiple RRAs in a single RRD

[Jim Breton]

Hi, I've been using RRDTool for about a day now, and I think it's great.

However I have a question.  I have noticed that in some sample
RRD-creating scripts I've seen online (including the one in the tutorial),
some people use more than one RRA in a single RRD.

Example from:
http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/tutorial/rrdtutorial.html

   rrdtool create test.rrd             \
            --start 920804400          \
            DS:speed:COUNTER:600:U:U   \
            RRA:AVERAGE:0.5:1:24       \
            RRA:AVERAGE:0.5:6:10

What is the advantage of having multiple archives (with different sample
intervals) if you are only graphing one of these?

For example, if I am graphing network i/o on a system and taking samples
every 5 minutes, why would I also want to create another RRA which takes
samples once per hour?  I realize that would make a useful graph as well,
but I don't see how to "get it out" of the RRA when you graph it, since
it's under the same Data Source; so as far as I can tell the only way to
determine hourly results based on the above RRD would be to extrapolate
your 5-minute samples.  But I'm sure I must be missing something!

Thanks.

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