[rrd-users] Re: Wrapping counter

[Alex van den Bogaerdt]

On Wed, Apr 23, 2003 at 07:04:40PM +0200, Bas Rijniersce wrote:

> >From the manual:
> 
> DERIVE
> will store the derivative of the line going from the last to the current
> value of the data source. This can be useful for gauges, for example, to
> measure the rate of people entering or leaving a room. Internally,
> derive works exactly like COUNTER but without overflow checks. So if
> your counter does not reset at 32 or 64 bit you might want to use DERIVE
> and combine it with a MIN value of 0. 
> 
> I read "derive works exactly like COUNTER but without overflow checks"
> as something that is oposite to what I want.. Or did I misread then?

Problem:

1) Counter is reset
2) RRDtool finds that the current value is lower than the previous value
3) There's only one conclusion, the counter wrapped (not!)
4) The increase of the counter is huge, the resulting rate is huge

Now the same, using DERIVE in stead of COUNTER:

1) Counter is reset
2) RRDtool is okay with counters going back
3) The increase is negative
4) The resulting rate is negative

Now the same, using DERIVE but this time with a minimum allowed value of 0:

1) Counter is reset
2) RRDtool is okay with counters going back
3) The increase is negative which isn't allowed -->  UNKNOWN
4) The resulting rate is UNKNOWN

This is what you want^Wneed.

If possible, do an update as soon as you find out the counter is reset. This
way RRDtool has a starting point for the next update (current-previous will
be unknown if previous is unknown).

HTH
Alex
-- 
Much of what looks like rudeness in hacker circles is not intended to give
offence. Rather, it's the product of the direct, cut-through-the-bullshit
communications style that is natural to people who are more concerned about
solving problems than making others feel warm and fuzzy.

http://www.tuxedo.org/~esr/faqs/smart-questions.html

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