[mrtg] Re: Jagged graphs above 500M/sec

[tony bourke]

If you are measuring every minute, then the 32 bit limit that you
experience is going to be 115 Mbps x 5, or a bit over 500 Mbps.  The only
solution at that point is to switch to 64 bit counters.

Here is a brief explanation of the trouble you are seeing.  Since you are
running every minute instead of the usual 5 minute interval, the limit is
multiplied by 5 in your case.


--------
One limitation to keep in mind is that the default configuration file
generated by MRTG cannot accurately measure bandwidth above about 114 Mbps
(Megabits per second). The reason is that the default interface counter is
a 32-bit number, and a 32-bit number will rollover twice in a period of 5
minutes at more than 114 Mbps. (232 = 4,294,967,296 bytes, multiplied by 8
(bits in a byte), divided by 300 seconds in 5 minutes, is 114,532,641 bits
per second, or roughly 114 Mbps.) MRTG or any other SNMP program cannot
correctly account for a double rollover. The graph will appear to peak at
about 115 Mbps and never go beyond that, when in reality the interface
could be running well beyond that rate. To get around this, you must
change the configuration file to poll a special 64-bit interface counter.


Hope that helps,

Tony


On Mon, 22 Oct 2001, Nick J. wrote:

>
> Hello,
>
> I have a MRTG graph set up to monitor aggregate usage of approximately 20
> high speed links (Mostly OC3's). Once the graph gets above 500M/sec, it gets
> very jagged and graphs some strange things. Once it drops below 500M/sec, it
> returns to normal again. I remember reading something once about problems
> with "integer wrapping". Is this what I'm seeing here? Is there any way
> around this? My cronjob is set to run every minute, on the advice of someone
> else to try and avoid the integer wrapping problem, and to try and keep
> accurate graphs, due to the amount of traffic being graphed. I've searched
> the mailing list archives with no luck.
>
> Any help would be appreciated.
>
> Thanks,
>
> -Nick
>
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Tony Bourke				tony at vegan.net



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