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

BAARDA, Don don.baarda at baesystems.com
Wed Aug 23 09:10:11 MEST 2000

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...

	But probably not worth the overhead...

> * 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
>   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.

	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.

	What does this all mean... nothing really, just theorising... :-)


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