[rrd-users] Re: counter or absolute

[Alex van den Bogaerdt]

On Wed, Dec 24, 2003 at 06:19:29AM -0500, Fauteux, Joe wrote:

> Ive read the definitions but am having a hard time determining what ABSOLUT=
> E does - If i want to graph, for example, CRC errors on an interface, a val=
> ue that may grow occasionally but often stays the same, should i use Absolu=
> te or Counter?  - either way it seems like i'll be seeing an empty graph mo=
> st of the time
> ideas?

First of all:  ABSOLUTE does NOT mean "the absolute value" of a counter.
I guess this is where the confusion comes from most of the times.

Secondly: RRDtool does RATES.  No matter what option you give, a rate is
a rate.  GAUGE ABSOLUTE and DERIVE tell RRDtool _how_ the input is to be
interpreted.

COUNTER: The value you get needs processing; subtract the previous value
         from the current one, then divide by the time lapsed.  If the
         rate becomes negative, the counter has wrapped so compensate
         for that fact.
         
DERIVE:  The value you get needs processing; subtract the previous value
         from the current one, then divide by the time lapsed.  If the
         rate becomes negative, that's fine with me.

ABSOLUTE:The value you get needs processing; you should divide the value
         by the time lapsed.  The value you got from the device is reset
         each time it is read so there's no need to do any other math.

GAUGE:   The value you get _is_ a rate, do not touch (at this stage of
         the process that is!)

(note that this is only part of the process.  More on the faq site)

Of course, nothing stops you from using "GAUGE" to display temperatures.
It will work, no problem.  You can also tell RRDtool to display interface
statistics as a GAUGE.  As long as you understand what's happening, you
can decide if that's what you want.

When graphing CRC errors, usually you do not want to know that you've
gotten 12 errors since startup (but sometimes you _do_ want to know!).
Usually, you want to know the amount of errors per time interval.  For
this, you should use a normal counter.  Using ABSOLUTE would mean to
RRDtool that the counter has increased from zero to its current value
in the current interval; all of those 12 errors are "seen" in the last
five minutes in stead of since startup.  Not what you want I guess.

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sure.

HTH
Alex
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