# [rrd-users] Re: About logging the REAL valuesn

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Tue Aug 22 10:57:23 MEST 2000

```Tobias Oetiker wrote:

> *> 2) disallow AVERAGE on this DST.  It is okay to use MIN,MAX and LAST.
>
[...]
> Using MAX and MIN is a different issue all together because rrdtool
> clearly defines that it FIRST builds an average value over the
> defined step interval and then takes the MIN and MAX of these
> average intervals ... If you need more precision (or higher peaks
> for that matter) you must have shorter sampling intervals ...

I have no problem with the average values.  When averages are used, you
simply *must* use the correct timestamp.  However, when logging minima
or maxima it is wrong (IMHO) to average the value first and then take
the minimum (maximum) of the averaged (altered!) value.
Shorter sample intervals help a bit here; they do not solve this problem.

The whole point is to define some DST that is valid during the whole
interval.  Normally when you enter data at Tx (T0 < Tx < T1) the value
is valid from T0..Tx (or is it: from Tx..T1?).  If the value would also
be valid from Tx..T1 (T0..Tx), I expect this to solve most problems.

Currently we advise users to enter data at timestamp-(timestamp%300),
this enters the data in the previous interval.  To mimic the behaviour
described in my suggestion, the data should be inserted at the next
timestamp:  timestamp+300-(timestamp%300).

There will be one issue with both of the ways to enter data.  What if
you enter two values during the same interval?  Using altered timestamps
will render it impossible to enter that second value.  With the altered
RRDtool we could define the behaviour.

regards,
--
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