[rrd-users] Re: rrdupdate not working properly?

Stefan Moser sm at open.ch
Wed Sep 6 16:15:13 MEST 2006

Hi Lejf,

ABSOLUTE, like DERIVED and COUNTER, are used for things that DO have a
relation to time. To make an example related to your memory graphs,
consider page swaps: The values for these are always in the context of
your polling frequency, e.g 'x page-outs in the last 300s'. Now you
COULD display these using a gauge and your graph would display the total
number of swaps per polling interval. However, from a sysadmin point of
view, it's much more interesting to know the rate, i.e. page-out/s.
Hence you'd use one of the counters ABSOLUTE, DERIVED AND COUNTER, which
of these depends on the type of counter you get your values from *.

ABSOLUTE is for counters that are reset with every read. The other two
are fundamentally the same: they are used for continous counters where
the value for a given period is current value - previous value. The
difference is in the way they handle counter overflows.


*) In the interest of full disclosure: For the image, you can convert
between rates and  total values by graphing a CDEF that
multiplies/divides the DEF by the polling period, instead of graphing
the DEF directly. But it usually makes more sense to just choose the
right DS type when creating the RRD.

Lejf Diecks wrote:
> Stefan Moser <sm at open.ch> wrote:
>> For stuff like that you use DS type GAUGE, not absolute. [..]
>> a metric like memory free "is not really connected with a
>> time". Hence you use GAUGE as a datatype.
> Hi Stefan,
> changing from ABSOLUTE to GAUGE works perfectly! I wonder what ABSOLUTE is good for?
> Anyway, thank you a lot!
> Lejf

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