[mrtg] Re: help with counter issue

[Alex van den Bogaerdt]

Roth, Gabrielle wrote:

> It seems to me from what I've read and seen on this list that this is
> probably a counter rollover issue.  The faq
> (http://faq.mrtg.org/FAQ004.html) says "32 bit will allow for a maximum
> value of 4294967295, and when divided by 300 (seconds) this is about 14
> megabytes per second or 114 megabits per second."  But, I'm getting less
> than 14 Mb:  when I add up the numbers from the sub-ifs, they add up to just
> under 4 Mb.  I'm guessing this is because the *max* is 14M, and mine is just
> showing less than that.  ???

The most important is the 2^32 value.  All other numbers in that faq-entry
are derived from that.

At intervals of 10 minutes (600 seconds) the rate at which a counter
overflow will happen *at each interval* will be 2^32 /600 which is
just over 7 MB or 57Mb.

At speeds less than 7MB, the counter will overflow less often.  In your
case it will be every other update.  Each missing update (caused by an
overflow) will be visible.

What you're probably missing is that even at 4MBps the counter will
overflow: Any start value, which incremented with 600 seconds at
4MBps becomes 2^32 or more will cause an overflow.  That means that
every start value of (2^32 - 600*4*10^6) or more (this is just over
1894 MB) will do so.  1894MB is approximately halfway the total
counter range...

Polling more often than once every 10 minutes will help in that it
will increase the amount of good intervals vs. counter wraps.

cheers,
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