From: "Wesley PA4WDH" <[email protected]>
To: <[email protected]>; "Alex van den Bogaerdt"
<[email protected]>
Sent: Sunday, February 17, 2013 1:53 PM
Subject: Re: [rrd-users] Strange behavior when using CDEF to divide two
values
> Hi Alex,
>
> At least i thought i did understand :-)
> I also see that the values stored in the RRD databases are not the exact
> values i store in it, as far as i know a result of normalisation.
Probably.
> The part i do not understand is that it may occour during graphing. The
> two datasources are stored in the same database and normalized so i
> wouldn't expect any normalization during graphing.
Correct. Normalizing occurs before the rates are stored in an RRA.
> I don't think consolidation will have any role in this situation since i'm
> only using one RRA to store a day of 5-minute samples, not the data is not
> consolidated any further.
For example: if you are using 1200 of those 5-minute samples, and are
graphing 600 pixels wide, RRDtool has no choice but to consolidate 2 samples
into 1, 600 times. Things become more complicated if you have start and end
times not on a logical boundary, or if (end-start)/300 is not a whole
multiple of horiz_pixels, and so on.
Dividing by _almost_zero_ does not produce NaN, it produces a very high
number. That is what you encountered. So this supports my hunch; you are not
looking at zero, you are looking at 1e-10, or you are looking at
{zero+something}/2. Or maybe {zero*3+something}/4, and so on.
Don't get me wrong, there is always the possibility of a bug, but that would
not be my first thing to investigate if I would tackle this problem.
For some it's a nice excercise to dive into this and analyse the situation.
For others it's a wast of time, and they should hire someone to do the job.
You know best in which category you fit best.
HTH
Alex
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