[rrd-developers] Re: (bug?) Incorrectly selecting low-res RRA

[Alex van den Bogaerdt]

On Fri, Mar 28, 2003 at 04:49:14PM +0000, Phil Radden wrote:

> This caused me to realise that the 'coverage before resolution' rule is 
> also applying at the beginning of time, not just the end of time, and that 
> my 'month' rra is only specified to include just the right amount of data 

Always consider what you ask RRDtool to do.  Try to match this
against the available data and see what you would do.

If you ask for a certain time period, and if RRDtool has this data
available (possibly fetching slightly more time to end up at a
boundary), it makes no sense to return a partially fulfilled request.

Hypothetical situation:

1 RRA in 5 minute intervals, 12 hours worth of data
1 RRA in 60 minute intervals, 24 hours worth of data

You request 00:15 to 23:45.

RRDtool has to choose:
- return 12:00 to 23:45, (missing 00:15 to 12:00)
- return 00:00 to 24:00, (slightly more)

I think it will return 00:00 to 24:00 here.

Next situation, same database:

You request 22:15 to 23:45.

RRDtool can satisfy this request.


Bordercase:

You request 12 hours, ending at 23:30

RRDtool has to choose:
- return 12:00 to 23:30 (missing 11:30 to 12:00)
- return 11:00 to 24:00 (includes 11:30 to 24:00, but last row will be unknown)

You request 12 hours, ending at 24:00

RRDtool can satisfy this request

You request 12 hours, ending at 23:55

RRDtool has to choose:
- return 12:00 to 23:55 (missing 11:55 to 12:00)
- return 11:00 to 23:00
- return 11:00 to 24:00 (one row will be unknown)

I don't know what RRDtool does do in this case. (and I can't be
bothered to check the source).

As you can see, it isn't that easy to come up with a proper solution.
I can imagine the RRA-selection algoritm doing things different from
what is expected, however I can also imagine users (like me and you)
expecting something different than that we specify.

To avoid problems I (almost) always make sure RRDtool can match my
request exactly.

HTH
Alex
-- 
Much of what looks like rudeness in hacker circles is not intended to give
offence. Rather, it's the product of the direct, cut-through-the-bullshit
communications style that is natural to people who are more concerned about
solving problems than making others feel warm and fuzzy.

http://www.tuxedo.org/~esr/faqs/smart-questions.html

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