[mrtg] With "options[target]: gauge" MRTG updates the RRD file with one value, but fetch returns a different (averaged) value

Daniel J McDonald dan.mcdonald at austinenergy.com
Fri Apr 20 14:28:22 CEST 2007

On Fri, 2007-04-20 at 05:51 -0500, Volk,Gregory B wrote:
> I'm using mrtg to plot the number of users associated with wireless
> access point over time. Even though the SNMP var is an integer, I've
> noticed non-integers in the data when looking at the output from
> "rrdtool dump", and "rrdtool fetch". It appears that there is some
> averaging going on inside the RRD file 

Correct, rrdtool always regularizes the data to n-minute norms, assuming
linear regression.  The goal is to determine what the data would have
been at precisely 06:15:00, rather than at 06:16:23.  The benefit of
this regularization is that your polling times can be a little off
(maybe you poll at 06:20:13, but then a problem with another host brings
in the next value at 06:28:15) but the stored data is predictable as to
the timestamp on the data.

> even though the GAUGE variable type was specified when the RRD file
> was created. It really doesn't make sense to do that in this case and
> fractional numbers of users (for the Peak and Current values embedded
> in the graphs) are causing the users who look at the data to call me
> and ask how 5.8 users can be attached to an AP.
> Is there anything I can do to disable the gauge averaging in this
> case? 

I don't believe so, but I'm not an rrdtool expert.

> I don't quite understand why it happens - gauge data doesn't really
> seem average-able. I set up a quick & dirty script (outside of MRTG)
> to verify what was happening and this is what I'm seeing. I don't know
> if it has always been this way or if this is the result of the version
> 3 rrd format. After five years of doing this you'd think I would have
> noticed it before now. ;)

mrtg/rrdtool has had this behavior for at least the past 8 years.  There
used to be a great FAQ on the topic.

Daniel J McDonald, CCIE # 2495, CISSP # 78281, CNX
Austin Energy

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