RRD introduction & examples


This section can be ignored if an openHAB backend is used. Help with displaying diagrams with the openHAB backend can be found in the documentation of the Diagram plugin.

General information about RRD

The CometVisu diagrams are generated via RRD. These are usually stored in the directory /var/www/rrd.

General information about RRD can be found in English here and of course at Wikipedia

Information on creating RRD databases can be found in English here

Information on updating / filling RRD databases can be found in English here

RRD and the CometVisu

To view a chart in the CometVisu you need:

  • CometVisu Chart Plugin
  • RRD file
  • rrdtool tuned to CometVisu (either an additional script or an RRDtool with patches)

(both are on the wiregate and on the Raspberry Pi image standard available)

root@Traumhaus:/var/www/rrd# cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the 'crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.


# m h dom mon dow user  command

# several rows away, please do NOT use this file copy & paste but attach the RRD Cron down to the existing File!!!``

*/5 *   * * *   root    /var/www/rrd/cputemp
*/5 *   * * *   root    /var/www/rrd/processes
*/5 *   * * *   root    /var/www/rrd/cpuload
*/5 *   * * *   root    /var/www/rrd/cpuload15

*/5 * * * * means that the job every 5 minutes at any time is performed. The smallest time interval with the crontab is realizable is */1, so every minute.

The update distance should match the RRD database!

Create the RRD database

The RRD file/database must first be created.

For this you have to consider when creating the database how many records should be stored. The database is then created and populated with “NaN” (Not A Number) values. The size of the database will then no longer change.

This means that when the maximum number of records in a table is reached, the oldest is rejected when a new value is entered. However, this is not a problem as values are regularly summarized.

In practice this means that the resolution of the values decreases with increasing time interval to “now”. (for example, minute values for the last 7 days, hourly values for the last 4 weeks, daily values for the last 2 years etc ...)

“Filling” the RRD database

The RRD file / database must be regularly filled with data - if this does not happen RRD recognizes this and enters automatic NaN - so that the summaries remain correct.

To do this, call the rrdtool with the “update” parameter and pass a time stamp (or N for “Now”), as well as the values to be included in the database.

The easiest way to do this is via a short shell script, which can then be executed regularly via cron.


If RRD-based data is not displayed as intended, you can look directly into the database to see if any (meaningful) values exist. There is this for that

rrdtool dump

Command. This represents the database as text. You should then filter out lines with NaN (Not a Number) and only look at data lines. This can be done e.g. with the grep command:

rrdtool dump NAMEOFTHEDATABASE.rrd | grep row | grep -v NaN

Examples of RRDs


These examples do not work with openHAB

(the example scripts generate the RRD database at the first call, then update the database)

for advanced users (adjustments in the script are probably required)