[rrd-users] Re: rrdtool theory ... (LONG)

Tobias Oetiker oetiker at ee.ethz.ch
Wed Aug 23 09:26:10 MEST 2000

Today you sent me mail regarding RE: [rrd-users] rrdtool theory ... (LONG):

*> G'day again...
*> > -----Original Message-----
*> > From:	Tobias Oetiker [SMTP:oetiker at ee.ethz.ch]
*> > Sent:	Wednesday, August 23, 2000 4:01 PM
*> > To:	rrd-users at list.ee.ethz.ch
*> > Subject:	[rrd-users] rrdtool theory ... (LONG)
*> > 
*> 	[...]
*> > * If you use rrdtool update several times in one step interval, ALL the
*> >   data you give to rrdtool will be taken into account. Internally rrdtool
*> >   counts the space below the curve built by the data points you feed it
*> > and
*> >   then when a interval time arrives it stores the accumulated space
*> > divided
*> >   by the interval time. This also takes unknowns into account which may
*> >   occur during the interval if frequent updates happen. (Alex: So yes, It
*> >   does help altering the sampling interval without altering the step)
*> > 
*> 	Interesting... This suggests that MAX, MIN, and LAST CF's _could_ be
*> implemented to have a different result to AVERAGE on a RRD with steps=1 and
*> more than one update in an interval. All it would take is to record the
*> min/max/last/first? sample area in addition to accumulating the total area.
*> Admittedly the data thus CF'd might be meaningless and confusing, but maybe
*> not that confusing and meaningless in the case of a GAUGE DS...

the whole point of the first averaging step is to come to a DEFINED
state from where you can work on ... as explained later in my
document, MAX alone has not realy much of a meaning in itself ... 

*> > * Once you have chosen an appropriate sampling interval you might also
*> > want
*> >   to look at the MAX data. Now here we run into another problem. Depending
*> >   on the nature of the data we monitor MAX always approaches 100% the
*> > shorter
*> >   the sampling interval is. (network traffic comes to mind) So even with
*> > MAX
*> >   consolidation it is important to know what the sampling interval was.
*> >   Taking the modem example: If over the course of the night, the MAX modem
*> >   use @ my Internet provider was 100% this sounds bad. But it is not the
*> >   full picture. Because the 100% are totally different when I know that
*> >   the MAX 30 minute AVERAGE was 100% or the MAX 1 minute average was 100%.
*> >   Together with the information that the AVERAGE use was 15% I can
*> > actually
*> >   build an opinion. The 100% alone is not interesting at all.
*> 	[...]
*> 	This is not entirely true in the case of a GAUGE. A GAUGE can have
*> an absolute maximum value which can be measured by a sample taken at the
*> appropriate peak. This is different to a COUNTER, for which a maximum rate
*> is only meaningful over a known time period.

as long as you do not look at the gauge ALL THE TIME, you can not
know that you catch the MAX ... 
*> 	When taking discrete samples, you miss potentialy interesting data
*> between the samples. This is heaps worse in the case of a GAUGE than a
*> counter... if your discrete sample happens to be taken at an
*> unrepresentative time, you get a totally false picture. At least with a
*> counter, you know the true average rate between samples. You could even
*> argue that min/max/average CF's are all meaningless using discrete samples
*> of a GAUGE.

this is just about knowing something about the frequency of the
signal on the gauge and choosing an apropriate sampling interval


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