[mrtg] Re: Spikes

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Thu Jan 24 02:00:02 MET 2002

Graham King wrote:

> I too get spikes sometimes when a device, monitored by MRTG using RRD,
> has been rebooted.

Can happen, has been discussed on the rrdtool users list.

> Am I right in thinking that any interface can show spikes when the
> monitored Device is reset, because the traffic counts will start
> again at zero and MRTG just thinks there's been a lot of traffic
> and the counters have wrapped round? If the resulting difference
> is less than MAXBYTES the you get a spurious spike on the graph.

Correct to the letter.

> If so, it would be nice to have an option that ignores data
> readings where this reading is less than the previous one, or
> maybe check the devices uptime?

The correct behaviour of the program inserting the data would be
to check uptime of the snmp counters.  Indeed one needs to check
that a wrap really is a wrap and not a reset.  When a reset has
occured, the value is unknown and exactly this has to be inserted
into rrdtool.

A hack that works well in most cases is to ignore all wraps, real
ones and fake ones, by using the DERIVE counter type.  DERIVE *can*
go below zero (so: no more wraps) and you can limit the minimum to
zero (ignoring those negative values).

Changing the counter type into DERIVE as well as changing the lower
limit into zero can be done using "rrdtool tune".

For counters that wrap often (such as 32-bit counters for 100mbps
interfaces) the hack doesn't work well enough (IMHO).  Use 64-bit
counters if possible (wraps are *very* improbible then) or do find
a way to check for fake wraps.

> -----Original Message-----
... should be above reply

 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
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