[rrd-users] Re: Howto collect information on two interfaces and put that information in one DS

[Alex van den Bogaerdt]

Lars Michael Jogback wrote:

> I wonder if it is possible to collect information from
> two router interfaces and in someway put that information
> in one database. It is the usual ifInOctets that I'd like
> to collect.

It has, just as collecting one DS, nothing to do with RRDtool.
RRDtool knows nothing about what you collect.  It just stores
rates, presented to it as numbers by another application.

You need to modify this other application and thus you need
to ask other people.

Anyway, if you add the counters each time, you will get what
you ask.

In the two following calculations we work with counters A and B.
The values are "n" and "p" which stand for "now" and "previous".
The rate calculated is R.  The sum is S.

Solution using CDEF:

  Ra = (An - Ap)/300
  Rb = (Bn - Bp)/300
  Rs = Ra+Rb

Solution using "your way":

  Rs = ( (An+Bn)-(Ap+Bp) ) / 300

Are these the same?

    ( (An-Ap) / 300 ) + ( (Bn-Bp) / 300 )
  = ( (An-Ap) + (Bn-Bp ) ) / 300
  = (  An-Ap  +  Bn-Bp ) / 300
  = (  An+Bn  -  Ap-Bp ) / 300
  = ( (An+Bn )-( Ap+Bp ) ) /300

So yes, ((An-Ap)/300) + ((Bn-Bp)/300) == ((An+Bn)-(Ap+Bp))/300

Of course you need to be careful for overflows, this will happen
more likely if you add two counters and then feed it to RRDtool.

HTH
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