[rrd-users] Re: Gauge understanding problem or error ?

[Alex van den Bogaerdt]

On Fri, Aug 19, 2005 at 12:11:39AM +0200, Christian Volkmann wrote:
> Hi Alex,
> 
> sorry for my confusing mail.I have some temperature values:
> 
> 12:34:20 20 degrees
> 12:35:20 30 degrees
> 12:36:20 40 degrees
> 12:37:20 30 degrees
> 12:38:20 20 degrees
> 12:39:20 0 degrees
> 12:40:20 0 degrees
> 12:41:20 80 degrees
> 12:42:20 0 degrees
> 12:43:20 20 degrees
> 
> I defined the database as GAUGE, step 60 seconds and try to record the MAX-values.
> I expected the graph to show the exact values from above.
> So the value of the graph should show at around 12:41 80 degrees.

Temperature from 12:39:20 to 12:40:20 is 0 degrees.
Temperature from 12:40:20 to 12:41:20 is 80 degrees.

Next thing RRDtool does is resampling of the data.  You wanted your data
in 60 second intervals so RRDtool makes sure it is in 60 second intervals.

(temperature up to 12:40 is 0 degrees)
temperature from 12:40:00 to 12:41:20 is 0 degrees
temperature from 12:40:20 to 12:41:00 is 80 degrees
(temperature after 12:31 is 80 degrees)

Temperature between 12:40 and 12:41 is:  20/60 * 0 + 40/60 * 80 = 53 1/3 degree.

> But the sample script shows about "54" degrees for "GAUGE", "step 60 seconds", MAX or LAST

Well, 53.3333333333333333333 is almost 54.

LAST and MAX (and AVERAGE and MIN) use resampled data.

> If I feed the database without ":20" as second, so ":00" I see the expected graph.
> 
> With CF=LAST or MAX:
> A) I never expected a graph with a value of 54 at 12:40
> B) I expect the same graph no matter if I record everything with ":20 seconds" or ":00 seconds"
>    for a step of 60 seconds. A time shift of the graph :20 or :00 would be correct.

Your expectations are wrong.  This is in the tutorial.  Read it.

If you want one-second precision, you should set "step" to be one
second.  Your database doesn't need to become bigger either, just
multiply the amount of steps per row (for each RRA) by 60.

Try the example with those new settings, MAX will work as you expect.
However, you will still not get graph intervals at HH:MM:20 because
your database is still in multiples of 60 seconds (thus: HH:MM:00).

Alex

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