[rrd-users] How are --start and --end supposed to behave?

Flavio Poletti flavio at polettix.it
Mon Oct 21 00:21:22 CEST 2013

Tobias Oetiker <tobi <at> oetiker.ch> writes:
> Hi Peter,

I hope it is not considered unpolite to reply to this very old post,
but I didn't find anything really applicable after this...

> what Alex says about time stamps I agree with (I think), so if you
> see data with a time stamp then this means that it is valid for
> the interval ending at that point in time so if the time stamp says
> 11:00 then the data associated with it is valid between 10:55 and
> 11:00 (assuming a 300 second step)

OK, if I get it the value returned by fetch or put in a graph at a
point in time represents what happened in the previous interval up
to that point in time. Makes sense.

> As for the amount of data fetch returns, the intended behavior is
> for it to return enough data to 'cover' the requested time
> interval at a resolution equal or better than requested ...

I don't fully get it. Reading through some tutorials (e.g.
http://www.vandenbogaerdt.nl/rrdtool/tutorial/rrdcreate.php) it
seems that it would be wise to calculate the number of items to
store in a RRA according to the number of pixels that we want in the
final image. So, being able to calculate the exact number of items
that a query would get can actually make a difference.

As I see it in 1.4.7, if I have:

--start a multiple of $step
--end=start+$(($N * $size))

I will get $N+1 items, so I should plan my graph to actually
be $N+1 pixels wide.

Should I upgrade to something different?



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