[rrd-users] Re: problem with manual reset - suggestion

Alex van den Bogaerdt alex at ergens.op.het.net
Mon Dec 9 15:08:28 MET 2002

On Mon, Dec 09, 2002 at 02:14:59PM +0100, Pressler Florian, DI wrote:

> > update using the "U" value for unknown.  RRDtool doesn't get to know
> > the exact value at reset time (or else you could do a regular update).
> > In other words: substitute your "R" for an "U" and it should work.
> It seems unlogical to me that this could work (I can't try it out at the
> moment). Example: If I cannot retrieve a value for whatever reason
> (e.g., because the UDP-packet containing the SNMP-Info is discarded), I
> would also have an "unknown" entry in the RRD. In this case, it would be
> unwanted if RRDtool assumed that the counter was reset, right?

No, not right.  The unknown entry prevents RRDtool from calculating
a wrong delta -> no problem when you reset the counter.

In other words: a counter reset needs an Unknown value.  An Unknown
value does NOT mean there was a counter reset.

> To my mind, there must be a difference between "value unknown" (which
> should lead to a gap, or interpolated data) and "value reset", which
> lets rrdtool know that the counter did NOT wrap, although it might seem
> so... How should RRDtool know, which of those options are true?

OK. Assume there is such an RESET.  What will the (part of the) interval
between te previous update and the current reset be?  Is it known? No,
it can't be known because you're not telling the current counter value.
Therefore the value will be unknown --> U.  The next time, RRDtool will
want to compute the next interval and can't do so (because
"current-previous" is "current-unknown" which gives unknown).  This can
be prevented:

Update at time "t-1" with "unknown".  Update at time "t" with "0" (you
*know* the counter is reset, right?).  The next (regular) update will
have a value and a time so Delta(value) is a known number and Delta(time)
is a known number --> no problem.

Reset doesn't belong in the back end.  It is a front end task.


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