[mrtg] Re: Weird MRTG/RRDTool/14all.cgi Behaviour
Alex van den Bogaerdt
alex at slot.hollandcasino.nl
Wed Feb 2 09:01:40 MET 2000
Jason D. Jordan wrote:
>
> What appears to occur is that the Y-Axis goes up into the 100mb range (peak
> at 120mb)and the graph changes to show only peaks rather than showing a
> constant yet varying quantity of data as expected.
You've hit a limit: the counters are 32-bit values and this only allows
for 2^32 bytes per interval. Each interval is 300 seconds so the maximum
rate possible will be 2^32 / 300 == 14316557 Bps or 114532456 bps.
This is best shown with some examples:
The number 4294967296 is 2^32.
time counter value
x 100
x+300 4294967395
In this case, the counter increased with 2^32-1. When added to 100,
this will not fit in a 32-bit value so you need to apply the mod function
to it: 4294967395 mod 2^32 == 99.
MRTG will first see 100, the next poll will show 99. This is not an increase
and MRTG assumes the device was reset (in this case: wrong assumption).
MRTG uses the previous rate, unless you have "Unknaszero" set.
time counter value
x 100
x+300 4294967397
In this case, the counter increased with 2^32+1. When added to 100,
this will not fit in a 32-bit value so you need to apply the mod function
to it: 4294967397 mod 2^32 == 101.
MRTG will first see 100, the next poll will show 101. This is an increase
of 1 and MRTG has no means of knowing it really is 2^32+1. It will use
value 1 and divided by 300 this is truncated to 0.
Increases above 2^32+299 will show on the graph as usual, albeit 2^32 too
low.
I'm not sure entirely but I think this is an SNMP_util problem and not
an MRTG problem. If you are able to collect 64-bit counters from your
device, you could use the external program interface and try it out.
Another workaround is to alter the size of the interval. As said:
the maximum possible increase is 2^32 per interval. Poll every minute
and the limit will be reached somewhere near 560 mbps.
Make sure to understand that after a counter wrap, MRTG will not use the
value!
Regards,
--
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