RRDtool is written by Tobias Oetiker <tobi@oetiker.ch> with contributions from many people all around the world. This document is written by Alex van den Bogaerdt <alex@vandenbogaerdt.nl> to help you understand what RRDtool is and what it can do for you.
The documentation provided with RRDtool can be too technical for some people. This tutorial is here to help you understand the basics of RRDtool. It should prepare you to read the documentation yourself. It also explains the general things about statistics with a focus on networking.
Please don't skip ahead in this document! The first part of this document explains the basics and may be boring. But if you don't understand the basics, the examples will not be as meaningful to you.
Sometimes things change. This example used to provide numbers like "0.04" in stead of "4.00000e-02". Those are really the same numbers, just written down differently. Don't be alarmed if a future version of rrdtool displays a slightly different form of output. The examples in this document are correct for version 1.2.0 of RRDtool.
Also, sometimes bugs do occur. They may also influence the outcome of the examples. Example speed4.png was suffering from this (the handling of unknown data in an if-statement was wrong). Normal data will be just fine (a bug in rrdtool wouldn't last long) but special cases like NaN, INF and so on may last a bit longer. Try another version if you can, or just live with it.
I fixed the speed4.png example (and added a note). There may be other examples which suffer from the same or a similar bug. Try to fix it yourself, which is a great excercise. But please do not submit your result as a fix to the source of this document. Discuss it on the user's list, or write to me.
RRDtool refers to Round Robin Database tool. Round robin is a technique that works with a fixed amount of data, and a pointer to the current element. Think of a circle with some dots plotted on the edge. These dots are the places where data can be stored. Draw an arrow from the center of the circle to one of the dots; this is the pointer. When the current data is read or written, the pointer moves to the next element. As we are on a circle there is neither a beginning nor an end, you can go on and on and on. After a while, all the available places will be used and the process automatically reuses old locations. This way, the dataset will not grow in size and therefore requires no maintenance. RRDtool works with Round Robin Databases (RRDs). It stores and retrieves data from them.
You name it, it will probably fit as long as it is some sort of time-series data. This means you have to be able to measure some value at several points in time and provide this information to RRDtool. If you can do this, RRDtool will be able to store it. The values must be numerical but don't have to be integers, as is the case with MRTG (the next section will give more details on this more specialized application).
Many examples below talk about SNMP which is an acronym for Simple Network Management Protocol. "Simple" refers to the protocol. It does not mean it is simple to manage or monitor a network. After working your way through this document, you should know enough to be able to understand what people are talking about. For now, just realize that SNMP can be used to query devices for the values of counters they keep. It is the value from those counters that we want to store in the RRD.
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