[mrtg] How configure mrtg for negative data?

[Steve Shipway]

> I try use mrtg for get temperature data in Celsius degree positive and
> negative. In positive all good, but negative is null. Ноw configure mrtg for
> negative degree?

If you want to use MRTG to graph negative numbers, then there are a couple of things you must do.

First of all, and most importantly, you need to be using RRDTool as the backend, not Native mode.  Native MRTG cannot support negative numbers (or decimals).

Next, you have to MANUALLY tune the .rrd files to set the new lower limit, as MRTG will only create RRD files with a minimum of 0.  To do this, identify the RRD files (they will be targetname.rrd in the WorkDir for a [targetname] target) and use 'rrdtool tune -i ds0:-10 -I ds1:-10' to set the minimum thresholds to -10 (or use another value of course).

You will possibly want to configure your web frontend to graph with a fixed Y-axis range; how to do this will depend on what you use.

Steve

Steve Shipway
T: +64 9 3737 599 ext 86487
E: s.shipway at auckland.ac.nz

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