[rrd-developers] problems graphing past end of RRD

[Tobias Oetiker]

Hi Thorsten,

droping end time matching seems like a very simple and good
solution to this problem ... will do ... thanks for the idea ...

cheers
tobi
Yesterday Thorsten von Eicken wrote:

> I'm having trouble when generating graphs that end after the last update
> time of an RRD. The problem is that I sometimes get a much coarser RRA
> than I should. The issue is that in rrd_fetch_fn there is code to
> determine the best partial match which kicks in when plotting past the
> end of the RRAs and if the step times of the RRAs aren't multiples of
> one-another strange things happen.
>
> The code calculates the matched time portion as follows (approx line 276
> of rrd_fetch.c):
>      /* best partial match */
>      tmp_match = full_match;
>      if (cal_start > *start)
>          tmp_match -= (cal_start - *start);
>      if (cal_end < *end)
>          tmp_match -= (*end - cal_end);
> I don't understand the purpose of calculating the end-time coverage. As
> far as I know, all RRAs end at the same time, modulo their respective
> step sizes. So they all cover up to the last update moment (again,
> modulo step size) and we should really exclusively focus on the start
> time coverage.
>
> Here is the abnormal situation I run into. I have RRAs with step sizes
> (in seconds): 20, 140, 1020, 4520, 13440, 53360. Suppose I need to plot
> a graph ending at 1203888560 when the last update time was 1203888537.
> The last points stored in the first 4 RRAs are:
>    20: 1203888537 - 1203888537 % 20 = 1203888520
>   140: 1203888537 - 1203888537 % 140 = 1203888420
> 1020: 1203888537 - 1203888537 % 1020 = 1203887640
> 4520: 1203888537 - 1203888537 % 4520 = 1203888440
> Now suppose the requested start time can be covered by the second (140
> second) RRA, you'd expect that to be chosen, but because the last point
> of the RRA @4520s step happens to be 20 seconds later you get that one
> instead!
>
> The net result is that depending on how the end times of the various
> RRAs line up you get a plot for the last day (that's what the 140 second
> step RRA is used in my case) that uses the 4520 second step RRA which is
> designed for the one month view! Not nice at all.
>
> I suggest removing the partial-match adjustment based on the end time,
> but maybe I'm missing a use-case where this is important.
> Regards,
> Thorsten
>
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-- 
Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten
http://it.oetiker.ch tobi at oetiker.ch ++41 62 213 9902