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

Alex van den Bogaerdt alex at ergens.op.het.net
Wed Jul 25 11:35:54 CEST 2007

> 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

"Nearest minute boundary":  No, it does not.  You tell it where to
do so, using "--step".

As told before, it is "n*step", not "n*step+offset". Thus, if you
want hh:mm:06, hh:(mm+1):06 and so on, your step size will need to
be a whole multiple of 1,2,3 or 6.

Yes, your create command may have used a wrong timestamp. RRDtool
will have fixed this.

Or, you have multiple RRAs, not all in the same resolution. Maybe
RRDtool had to select the RRA where each row describes 60 steps.

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

Normalization is explained on my site.

> As I said in an earlier mail message, I think rrd is real good at what 
> it does and I'm only trying to understand the areas in which someone is 
> trying to do something it really wasn't intended for and this sounds 
> like it may be on of those areas.

I still think RRDtool can do exactly what you want, provided that you
tell it what you want.  There may be cases where RRDtool cannot do what
you want it to do, but I still think your case isn't one of them.

Alex van den Bogaerdt

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