[mrtg] Re: Broken Graphs

[Alex van den Bogaerdt]

Brian E. Seppanen wrote:

> On Thu, 9 Nov 2000, Joseph Smith wrote:
> >
> > 	I am pulling stats on a few OC-3 interfaces and as soon as my
> > bandwidth hits about 120 megabits my graph just dies off,  not completely
> > just to 100kb to 1000kb. I know that my traffic is not actually dropping
> > off. Any thoughts Thanks
> 
> In my experience it has been due to the MaxBytes setting being too low.
> It's obvious when you look at the graph and it's kind of striped, and you
> noticed its striped only at the peak points.  Find out the true MaxBytes
> and restart mrtg.

Yes and no.  When using 32-bit counters, the maximum value of that
counter will be 2^32-1 (4294967295).  After that, the counter wraps
to 0.  If the traffic is just over 2^32 bytes/300 seconds ==
4294967296/300 == 14,316,557 Bps or 114,532,456 bps then the wrap
will have occured every time you run MRTG.  The result for 120mbps
is:

   counter value x at time t
   counter value x+120,000,000/8 at time t+300
convert bits to bytes
   counter value x+15,000,000 at time t+300
take the wrap into account
   counter value x+(15,000,000-14,316,557) at time t+300
which is
   counter value x+ 683,443 at time t+300

MRTG will only see an increase of 683,443 B in 300 seconds
which is 18kbps.  It has no way of knowing that the counter
wrapped (or: if there is a way MRTG doesn't know about it).

The solution is to use 64-bit counters.  A possible workaround
is to decrease the polling interval to, for instance, one minute.
No modification to the config file is needed to poll more often
however it may be necessary to switch to RRDtool for performance
reasons.

cheers,
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