[rrd-users] Re: disabling interpolation?

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Wed Aug 23 01:29:40 MEST 2000

Tobias Oetiker wrote:
> *> Either you measure at 9:00 and 9:05 *exactly* or you need another way to
> *> work around the problem.  If I enter "4" for the number of people in the
> *> room at 9:04, and the number used to be 5, I cannot say that there was
> *> a minimum of 4.80 people in the room...
> the point is that if you want to have higher accuracy and this is
> what this is basically about then you have to increasse the
> sampling rate ... because otherwhise it will be completely by
> chance that you catch the 4 people in the room at 9:04 while there
> were 5 people in the room at all other one minute points between
> 9:00 and 9:05 ... So in order to do this properly you have to set
> the sampling interval to 1 minute and measure the population every
> 60 seconds ... all the rest is complete randomness ... 

I assume you mean: alter the step time.  If not, it doesn't help to
alter the sampling interval (or does it?).

However, the problem is not in the sampling rate.  This other problem
which you described perfectly, should be handled by altering the sample
time and the step time.

The problem I'm talking about is still there, even in your example.  It
is just altered from 12:00..12:05 to 12:00..12:01.  The data will still
be entered not exactly at 12:01 and therefore the value actually stored
in the database will not be 4 or 5, it will be 4.8 (or any other random
number).  The problem is in the interpolation, not in the accuracy.

No matter what step time and which sampling rate you use, either you enter
the data at massaged time intervals (solving the problem in the front end)
or you define a new DST (solving the problem in the back end).

Solving this other problem is impossible with RRDtool;  No matter how
short the interval is, you are never able to make sure that you have
seen all values during that interval.  You can only monitor the value
at the sample point.  

I do see a workaround inside an RRD.  If the RRD contains a datapoint
for every second, the interpolation won't happen.  Using min and max
RRA's is now possible.  Of course, this work around will only work
while a second is the smallest step time possible.

 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
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