[rrd-users] Re: Manipulating and Graphing Data

[Alex van den Bogaerdt]

On Tue, Oct 31, 2006 at 03:10:41PM -0600, Tony Varriale wrote:


> > GAUGE is _not_ a value, it is a rate.  And it is subject to
> > any normalization and consolidation like any other rate.
> 
> Well...I'm really confused.  Here is what it's states from the beginner's 
> guide:

> "GAUGE does not save the rate of change. It saves the actual value itself. "

Yeah.  Except that this is oversimplified.  Really, you can find more
information on normalization and consolidation on my site.


Stop thinking "n drops".  Start thinking "n drops per second".

Everything (!!!) is a rate in rrdtool, and I don't care what
the beginners guide says. It is wrong.  Contact its author
to get this fixed.


Inputting "1" as derive or counter, when the previous input was
"0", means "1 per 300 seconds" (assuming 300 second interval).

But inputting "1" as gauge means "1 per second", not just "1" and
also not "1/300".  This is what is used "as is".  But if the interval
wasn't 300 seconds (but 305, or 295) then normalization will still
occur.  That's why inputting "1" may result in "0.93" or "1.07".

This "1" is NOT stored.  If the interval happened to be exactly
300 seconds and if it started exactly at the right time, then
the resulting normalized rate will also be "1".  _That_ "1" _is_
stored, but that value could also have become 0.93 or 1.07 or...


Please, do yourself, me and the rest a favor and first read the
available information before making wrong assumptions again.  I
wrote it for a reason, namely so that I did not have to repeat
myself each time on this list...  which brings me to searching
the archives...  use them.  You can read questions and answers
you have not yet asked.


Now, why DERIVE:

Changing from 1 to 0, in a 300-second interval, and using DERIVE:

DERIVE would make this: 1->0 in 300 seconds ==> -1/300

Indeed, the rate changed; you have 1 drops less per 300 seconds.

"5" to "1" -> -4/300 drops per second.

You expect -4 ? Then you need to multiply by 300.  That would
make it a value per 300 seconds (NOT just 4).

You also want to remember 1 and 0, or 5 and 1 ?  Then you also
want to use GAUGE.  But be careful, because you would be entering
a rate that is approximately 300 times too high.

-- 
Alex van den Bogaerdt
http://www.vandenbogaerdt.nl/rrdtool/

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