[rrd-users] A reason there isn't a CF SUM function?

[Derek Haynes]

Tobias,

> in any event, on a technical level, what would be required to
> achieve what you want (appart for the handling of unknown periodes
> within a single step) would be an AVERAGE_UNKNOWN_AS_ZERO
> consolidation function ... this would not break the assumptions
> about the nature of RRAs and you could calculate your type of SUM
> easily ...

Makes sense.

>   a + unknown = a

This seems to have been addressed in other areas (ADDNAN / TOTAL
w/VDEF) both appear to treat unknowns as zero. An AVERAGENAN operator
for a CDEF would seem to complete the loop.

Any suggested starting points for adding this to rrdtool? I haven't
dug through the source before.

Cheers,

Derek

On Wed, Aug 22, 2012 at 2:50 AM, Tobias Oetiker <tobi at oetiker.ch> wrote:
> Hi Derek,
>
> Yesterday Derek Haynes wrote:
>
>> Several times on the list, the topic of a CF SUM function has
>> risen...just a sample:
>>
>> http://oss.oetiker.ch/rrdtool/forum.en.html#nabble-td1059458
>> http://oss.oetiker.ch/rrdtool/forum.en.html#nabble-td1059458
>> http://oss.oetiker.ch/rrdtool/forum.en.html#nabble-td1059458
>>
>> I'm in need of a similar CF SUM function and I was curious if there
>> isn't one because (1) there's a fundemental reason why it's not
>> provided or (2) it just hasn't been implemented.
>
> the fundamental problem with a SUM CF is that the values of the RRA
> entries would then depend on the step interval of the RRA and thus
> fundamentally change the nature of the RRAs ...
>
>> For reason (1), the common argument is you can calculate this by
>> multiplying the average by the duration of the CDP. However, this
>> doesn't hold when a CDP contains unknowns as the average is only based
>> on the known values.
>
> basing a sum on unknown values might be problematic too
>
>   a + unknown = a
>
> would mean that you assume that unknown equals 0
>
> where we curently make the assumtion that the unknown part of the
> periode was similar to the known parts ...
>
> in any event, on a technical level, what would be required to
> achieve what you want (appart for the handling of unknown periodes
> within a single step) would be an AVERAGE_UNKNOWN_AS_ZERO
> consolidation function ... this would not break the assumptions
> about the nature of RRAs and you could calculate your type of SUM
> easily ...
>
> cheers
> tobi
>
>
>> The TOTAL operator does exactly what I'm looking for but it's only for
>> VDEF statements.
>
>
>> Any thoughts?
>>
>> Derek
>>
>>
>
> --
> Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
> http://it.oetiker.ch tobi at oetiker.ch ++41 62 775 9902 / sb: -9900



-- 
Derek Haynes
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