[rrd-users] Re: few questions about max value in rrd database

Serge Maandag serge.maandag at staff.zeelandnet.nl
Mon Mar 1 12:13:05 MET 2004

> I have a 64kbps leased line I get the counters from router in bytes, 
> I represent them on graph in bits/sec, using CDEFs.
> The max value 65000 (for input and output) in the rrd 
> database, is this correct (so the correct value in bits)? 

No, if the rrd holds bytes, your max should be in bytes.

> What happens when my leased line is upgraded by the provider 
> eq. to 128 kbps?

you will get NaNs in your database when you use more than 64k
on the line.

> How to change the max
> value? Should i dump the rrd in .xml format and change it 
> manually or should i use the "rrdtool tune"

rrdtool tune -a ds0:16000

> to change the max value. My experience is when i use rrdtool 
> tune on an existing rrd database the data collection stops. 
> What did I wrong?

Perhaps you set the max too low? I used rrdtool tune a couple 
of times and it has never failed me.

> Or set up a quite huge max value eq.: 100000000 (100 Mbit). 

If you want no max value, you can set it to U.

> And using CDEFs like this:
> CDEF:cdefa=a,8,*,65000,GT,65000,a,8,*,IF  or 
> CDEF:cdefa=a,8,*,65000,GT,UNKN,a,8,*,IF
> Which way is the better (I mean set the correct max value or 
> using CDEFs)? 

That's entirely a matter of tastes. 
First of all, when setting a max value or when cropping with cdefs
it's best to use a value that's a bit higher than the max you are 
expecting. Otherwise you will get NaNs when you updated the database
a few seconds too early or too late.

Secondly, max values are a good option, but they need to maintained.
I frequently have lines that get upgraded, so I prefer the cdef way.
It needs some code though, setting max values does not.


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