[rrd-users] Re: graph shooting up

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Sun Aug 6 12:31:45 MEST 2000

Mark Manuel C. Ramos wrote:

> Alex van den Bogaerdt wrote:
> > What are you monitoring?
> > 
> > For normal ethernet graphs, there should not be a huge peak when a
> > device cannot be polled.  There should be an unknown in the database.

> I'm monitoring a frame relay circuit, using rrdtool version 1.024.  What I
> want to happen is when I cannot reach the device (e.g. timeout via snmp).
> The graph will drop to zero.  I tried updating it with "NAN", "0" and "U".
> But nothing works.  The graph still shoots up to 80M sometimes G.

You have to check a few things.  First, make sure you understand what's

1) You poll the device
2) the value is fed into RRDtool
3) RRDtool does some calculations with the value

You are monitoring a counter.  The delta is calculated by RRDtool as follows:
(current counter)-(previous counter)

Suppose you enter "0" for the value.  RRDtool will calculate the delta.
It will be negative:  0 - <some number>.  RRDtool assumes a counter wrap
and adds 2^32 to the value --> huge positive value.
This is not the way to go.

Suppose you enter "U" for the value.  RRDtool will process this unknown
value.  Depending on your xff value (see rrdcreate.txt) this unknown will
make it into the archive.
This is what you need.

I'm not sure if you can enter NaN for the value.  You shouldn't do so.

Once you've got the data collecting part right you can move on to the
data graphing part.  It makes no sense to try and graph wrong data.

You have another problem.  You forgot to set the min and max values to
some sane values.  See rrdcreate.txt and rrdtune.txt for the why and the

Try to fill a database by hand with some known numbers.  Dump the database
inbetween each update (using "rrdtool fetch") and see what happens.
Perhaps you will find out why RRDtool is doing something that you don't

If your database contains correct values you might occasionally see
unknown slots.  When you graph this data, you should see a gap in the
graph.  If you want to modify these unknowns into a number, you have
to use "CDEF" in your graphing script.  This is explained in the docs,
and in my tutorial.

 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
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| http://faq.mrtg.org/                                                 |
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