[rrd-users] newbie rrd question - probably a very old issue

Tobias Oetiker tobi at oetiker.ch
Sat Sep 1 17:14:33 CEST 2007

Dear Dan,

> I think you're missing the point.

just take a look at the number of posts alex makes. he does
understand how rrdtool works, he know the code that makes it tick
... so if you tell him he is missing the point, chances are that he
will not bother anymore help you understand how rrdtool works ...

to your question.

RRDtool tries hard to work correctly independant of the number of
points it has to work with. So your graph, should turn out the same
results, regardles of the number of samples that are available
(withing reason).

This means for example, that if you input data from an RRA that is
using AVERAGE consolidation, that the graph will also use AVERAGE
consolidation to reduce the data until it fits in the graph. If you
use a MAX function over this, the MAX will return what you see in
the graph.

If you are interested in keeping the MAX of the data you have
originally fed into RRDtool, you must store it in an RRA that has a
MAX function attached.

A little example:

You have to 2 points in your RRA

a: 10
b: 90

On the graph they map onto the same pixel. Depending on the
consolidation function of the RRA, rrdtool will draw the pixle at
the following positions

avg: 50
min: 10
max: 90

on top of this you can use VDEF to calculate, but obviously the
results will depend on the first round of consolidation that has
already happened.

>From a performance point of view, given your box has enough memory,
you should be able to get about

 * 200-400 RRA updates a second per spindle

Note that you can cheat this by increassing the amount of dirty
buffers the OS tolerates as well as the ammount of time the dirty
buffers can stay dirty.

Especially if you run frequent updates this can be a neat strategy
to increasse throughput dramatically (in cache updates, rrdtool 1.3
can do about 20k per second on a current box.)


Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten
http://it.oetiker.ch tobi at oetiker.ch ++41 62 213 9902

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