The idea of a RRD is a fixed number of sample data points in a given database. The size of the database is defined at its creation time. The advantage of such a database is that the space allocated is fixed, and there s no danger of running out of storage space, ever; hence it is ideal for embedded / low resource systems.
Creating a database
rrdtool create eth0.rrd \ --start now \ --step 60 \ DS:ina:GAUGE:120:U:U \ DS:outa:GAUGE:120:U:U \ DS:in:DERIVE:120:U:U \ DS:out:DERIVE:120:U:U \ RRA:LAST:0.5:1:10 RRA:AVERAGE:0.5:5:10eth0.rrd is the name of the database (saved filename)
--start means when do we start populating the database
--step means sample size in seconds ( enter data every minute - will get a cron job to do this)
DS - definition of a dataset: label : type : heartbeat : lower limit : upper limit
type can be of :
GAUGE - save the value vithout any modification
COUNTER - save the rate (current - last) / sampletime
DERIVE - same as about with over/underflow adjust
ABSOLUTE - for data which gets reset when read
The key thing to remember is, the data gets modified before saving, for counter,derive types by rrdtool. Counter / Derive is ideal for saving data such as bitrate - as it is computed at ever sampling point, and then saved.
In above database each data entry point will need 4 bits of data (= number of DS definitions)
First two data will be saved as absolute values, next two will be saved as rate values. (in this case it will only start updating database after 2 sets of data has received. We the also define two Round Robin Archives :
RRA:LAST:0.5:1:10 - holds 10 absolute values, updated every minute
RRA:AVERAGE:0.5:5:10 - holds 10 values, averaged every 5 minutes
Instead of creating multiple RRA's one can have a single RRA , with 1 minute resolution.
The following will create a foo.rra which hold an year's worth of data in 1 minute resolution:
rrdtool create foo.rrd \ --start now \ --step 60 \ DS:in:DERIVE:120:U:U \ DS:out:DERIVE:120:U:U \ RRA:LAST:0.5:1:525600
and the datebase will only take 8MB of space -Incredibly efficient !
As defined in the DS section, we must insert 4 data values at every minute (at every sample point). The command to do so is:
rrdtool update eth0.rrd N:<value>:<value>:<value>:<value>
Dumping out the contents of RRD
rrdtool dump eth0.rrd will dump out the contents in xml
Creating pretty graphs ..