[rrd-users] trying to understand the relationship between source data, what's in rrd and what gets plotted

Simon Hobson linux at thehobsons.co.uk
Tue Jul 24 21:58:54 CEST 2007

Mark Seger wrote:
>I tried what you suggested by using a min/max RRA in addition to storing
>all the data in a 3rd RRA and it appears to work as you suggested it
>would, but some of the peaks aren't as high as I expected and so I did
>some further digging to try and understand what I'm seeing.  I have
>samples that were taken every 10 seconds starting at 16 seconds past the
>minute.  On my command to create the rrd database, I have:
>--start 1123992076  --end 1124076920
>noting that the sampling time drifted 4 seconds over the course of the
>day, something I've since fixed in my data collector and so it now
>collects samples without any drift and on multiples of an even minute so
>this shouldn't be an issue in the future.
>In any event, when I look at the contents of the rrd database that
>contains a day's worth of 10 second samples using fetch. starting at
>1123992076  I see the first interval at 1123992080, which leads me to a
>couple of questions:
>- does rrd choose to normalize data to the nearest minute boundary and
>therefore I get timestamps of 1:00, 1:10, 1:20 even if I enter data as
>1:06, 1:16, etc?  keep in mind my start time for the create DOES land on
>a 6 second bounday

See earlier posts in this thread, boundaries are ALWAYS on a multiple 
of your step size from unix epoch. Eg, is divisible by 10, 1123992076 
is not.

>- assuming it does pick its own intervals, is that why none of the
>numbers stored in rrd match the source data even though I have one row
>per sample, because it's normalizing the data?

Correct, as already mentioned it is normalising the data - but it 
does not choose it's own periods, it uses what you tell it when you 
set the step size.

>- if that's the case, I suppose that would explain why I'm not seeing
>the right numbers in my plots.


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