RRDtool in short....

Copied notes from: http://oss.oetiker.ch/rrdtool/tut/rrdtutorial.en.html
- RRDtool refers to Round Robin Database tool.
- RRDtool works with with Round Robin Databases (RRDs). It stores and retrieves data from them.
- RRDtool stores all sort of time-series data.
- If you measure some value at several points in time and provide this information to RRDtool. RRDtool will be able to store it.
- RRDtool originated from MRTG.
- RRDtool create graphs in PNG formats.
- We only need sensors to measure the data and able to feed the data into RRDtool.
- Create sample Round Robin Database,
rrdtool create test.rrd --start 920804400 DS:speed:COUNTER:600:U:U RRA:AVERAGE:0.5:1:24 RRA:AVERAGE:0.5:6:10
- Update database,
rrdtool update test.rrd 920804700:12345 920805000:12357 920805300:12363
rrdtool update test.rrd 920805600:12363 920805900:12363 920806200:12373
- fetch data from database,
rrdtool fetch test.rrd AVERAGE --start 920804400 --end 920809200
- "NaN" stands for "Not A Number" - Something is wrong.
- "UNKN" stands for "UNKNOWN", which is ok.
- Create graphics,
rrdtool graph speed.png --start 920804400 --end 920808000 DEF:myspeed=test.rrd:speed:AVERAGE LINE2:myspeed#FF0000

There are several ways for RRDtool to get a rate from its input:
* GAUGE: keep it "as is". The input is already a rate. An example would be a speedometer. This is also the type used for keeping track of temperature and such.

* COUNTER: look at the difference between the previous value and the current value (the delta). An example would be an odometer. The rate is computed as: delta(counter) divided by delta(time).

* ABSOLUTE: as the odometer, but now the counter is reset every time it is read. Computed as: value divided by delta(time).

* DERIVE: as COUNTER, but now it can also go back. An example could be something monitoring a bidirectional pump. The resulting rate can be negative as well as positive.