[rrd-users] Re: suspicious interpolation

[Tobias Oetiker]

Today Philippe Charnier wrote:

> Salut,
>
> Tobias Oetiker <oetiker at ee.ethz.ch> wrote:
>
> >rrdtool assumes a GAUGE value to remain the same from one update to
> >the next.
> >
>
> by putting 1000000000:2 1000000009:2 1000000020:4, I have something like:
>
>  4            |----------*
>               |          |
>  2   *--------*          |       [pdp_new]
>      | [18]   |   [44]   |       * data point
>   ---+---------+---------+--     + pdp_st
>  ...00     ...10     ...20
>
> (...00, ...10 and ...20 are the pdp_st of my example)
>
> If the value remains the same until the next update, I would expect
> the second pdp_new to be [11*2=22] and not [44] which takes the next
> value (4) into account.  That will make a result of 2 for ...10, a
> more correct value than 3.1.
>
> If I enter ...20:4, I would expect ...20:4 when fetching the results
> (i.e values entered at correct pdp_st should stay the same (in the
> GAUGE case at least).

Hi Philippe,

your drawing depicts the operation of rrdtool ... it does not
assume a slope but rather that the data value of a GAUGE datasource
has the sane "new" value from the point in time the last value war
recorded to the point in time when the new value is enterd ...

cheers
tobi

> Have a nice day
>
> ------                                                            ------
> Philippe Charnier          charnier@{xp11.frmug.org,free.fr,FreeBSD.org}
>
>     ``a PC not running FreeBSD is like a venusian with no tentacles''
> ------------------------------------------------------------------------
>

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