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Changing the timezone of cacti graphs using rrdtool

Posted in Linux, Work on April 6th, 2010 by Nicolas

I’ve recently come accross an interesting problem while trying to use a cacti install running on a server located in Europe in order to monitor, generate and export statistics to be read by people in Central America. Indeed, the generated graphs indicated CET time while the people for whom the graphs were interested expected UTC-6 time.

While there has been a support request in cacti for this particular feature and I’ve come across a patch for 0.8.6 on cacti forums, I haven’t found a solution that is integrated into cacti. So I went for an external graph generation script.

Cacti generates graphs by invoking rrdtool which itself relies on the value of the TZ environment variable to determine the offset that it must apply to variables stored inside the database. Indeed, the time stored in an rrd is UTC by default and an offset is applied during graph generation in order to transpose this to any local time according to the value of the TZ variable.

The script I’ve created simply uses the command line I got from cacti by turning graph debugging on in the graph management for the particular graph I wanted to export.

TZ=”America/El_Salvador” /usr/bin/rrdtool graph – –imgformat=PNG –start=-86400 –end=-300 –title=”Radio Clasica” –base=1000 –height=120 –width=500 –alt-autoscale-max –lower-limit=0 –vertical-label=”” –slope-mode –font TITLE:12: –font AXIS:8: –font LEGEND:10: –font UNIT:8: DEF:a=”/var/lib/cacti/rra/paris_clasica_91.rrd”:clasica:AVERAGE AREA:a#AFECEDFF:”” > clasica_1.png
TZ="America/El_Salvador" /usr/bin/rrdtool graph -
--imgformat=PNG --start=-86400 --end=-300
--title="My graph title" --base=1000 --height=120
--width=500 --alt-autoscale-max --lower-limit=0
--vertical-label="" --slope-mode --font TITLE:12:
--font AXIS:8: --font LEGEND:10: --font UNIT:8:
DEF:a="/var/lib/cacti/rra/my_file.rrd":somefield:AVERAGE
AREA:a#AFECEDFF:"" > somefield_1.png

The TZ=”America/El_Salvador” part of the command line redefines the value of the TZ environment variable before executing rrdtool. This modification is only local to the process from which rrdtool is launched and does not affect the current shell. The TZ variable is part of the zoneinfo package, the full list of possible values for this variable can be found under the /usr/share/zoneinfo directory of any linux machine.

Notice the –start=-86400 –end=-300 part in the above command, these indicate respectively the start and end time for the graph to be generated. The values above correspond to a full 24 hours (the day view in cacti). For the week, month and year view, the values are as follows:
--start=-604800 --end=-1800
--start=-2678400  --end=-7200
--start=-33053184  --end=-86400
The last step for me was to add this to crontab, I’ve created a file in /etc/cron.d which I named graphExport with the following contents:
MAILTO=myusername
*/5 * * * * www-data /path/to/myscript.sh >/dev/null 2>&1
www-data is the user who owns the rrd that is used for generating the graphs by myscript.sh.

Using this trick, I can now generate graphs that make sense to the people they’re intended for without asking them to perform the conversion from UTC by themselves.

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