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

Alex van den Bogaerdt alex at ergens.op.het.net
Fri Aug 19 13:49:17 MEST 2005

On Fri, Aug 19, 2005 at 10:38:34AM +0200, Christian Volkmann wrote:

> thanks for the explanation. I do not see a proper explanation of the CF "LAST" or "MAX"
> at the tutorial which describe a resampling.

You don't see "Data Resampling" ?

> Some design I do not understand.
> - Why does a GAUGE do a resampling for "MAX" and "LAST".
>   For me a maximum in a period should never be calculated down
>   to an average.

data is ___ALWAYS___ resampled to fit in a time slot.  YOU are asking
rrdtool to do that.

> - Followed up by this design the two pictures created by the script
>   look very different.
>   The only difference from the recorded values is recorded with
>   20 seconds time difference.
>   But that should not affect "MAX" nor "LAST".

Well, have it your way and do as I asked you to do.

> The resampling/consolidation expected for MAX should be:
>   if ( "in time period" && newvalue > "value of time period" )
>      "value of time period" = newvalue;
> The resampling/consolidation expected for LAST should be:
>   if ( "in time period"  )
>      "value of time period" = newvalue;

Resampling:    no.
Consolidation: yes.

Don't write it like it's the same.  Resampling and consolidation are
not only different but also happen in a different stage.  MAX, MIN,
LAST and AVERAGE work on rates.  Rates are resampled to match YOUR
requested step size.

And no, you won't have a rate until you have a complete timeslot.
RRDtool cannot look into the future, only into the past.  It will
not and should not use the most current value, it will use the most
current time slot.

> Otherwise the name "MAX" or "LAST" confuse me.

That is because you have your mind set to a certain assumption and even
if someone tells you that assumption isn't right, you cannot let go.

MAX rate. LAST rate.  The rate you get is 53.333333
The last (or: max) rate seen is 53.33333

> Isn't the "MAX" "80" to be recorded, especially for "MAX" ?

What do you think.  I have made the computation by hand, especially
for you, and end up with 53.333333  Do you think both RRDtool and me
are wrong while you are right ?

Maybe you should try and understand why the rate is 53.3333 instead of
repeating that it should be 80.  It's not like it hasn't been discussed
on this list before.

> Think about somebody using it to record the maximum temperature.
> He has seen "80 degrees" on the thermometer. has recorded it with
> rrd, looks at the history and just see 53,3 degrees in his records.

I have asked you to do that experiment with step==1.  Do it.

Don't continue to say how rrdtool should behave. It won't, unless
you understand what you are doing and give it the right commands.


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