# [rrd-users] Re: RRD Usage Query

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Wed Feb 21 02:10:31 MET 2001

```Dan McGinn-Combs wrote:

> What I'm concerned about is the interpolation of the the Elapsed times. If I
> the sent time is off, how does RRDTool interpolate on a GAUGE.... or does
> it?

Using gauge simply means entering a rate into RRD.  If you do so at
any moment in time not being the border between two intervals, you
are defining both the time before the border and a portion of the
interval starting on this border.  The next update will define the
rest of this interval.  The two rates are interpolated.

Make sure you view this picture in a fixed width font:

---B1-----B2-----B3-----
^      ^      ^
U1     U2     U3

The first half of the interval (B1->B2) is build from the rate entered
at U1.  The second half of interval (B1->B2) is build from the rate
entered at U2.  The interpolated rate for interval (B1->B2) is therefore
(U1+U2)/2.

If your updates are not exactly halfway the boundaries, the ratio U1:U2
also changes.

---B1-----B2-----B3-----
^      ^      ^
U1     U2     U3
For instance, if updates occur one minute after the boundaries and
each interval is five minutes, only 1/5 of interval (B1->B2) is build
from the rate at U2 and 4/5 of that interval comes from the rate at U2.

Updating exactly at the interval means 0/5 times U1 and 5/5 times U2.

cheers,
--
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