[rrd-users] Re: MAX value differs on daily, weekly, monthly and yearly graphs.

Alex van den Bogaerdt alex at ergens.op.het.net
Thu Sep 1 20:36:19 MEST 2005

On Thu, Sep 01, 2005 at 07:10:18PM +0200, Simone Morandini wrote:

> I'm having a little problem when graphing the bandwidth of our network,
> in particular for the MAX value.
> I have 4 graphs: daily, weekly, monthly and yearly, updated,
> respectively, every 5 minutes, 30 minutes, 2 hours and 24 hours; the
> number of primary data points is 1, 6, 24 and 288.

PDPs are computed from your input.  They are rates during a normalized
time interval: 300 seconds in your case; starting just after time
n * 300 and ending at time (n+1)*300.  Those PDPs are then used to
fill the RRAs.

> What I'd expect to happen, supposing that at a given moment a new
> maximum occurs and remains for at least 24h, is that after 30 minutes
> the weekly graph displays that value, after 2 hours the monthly graph
> does the same, and so the yearly one after 24 hours, but this doesn't
> happen. Plotted values are similar, but not exactly the same.

RRDtool doesn't take the last 288 PDPs to compute one CDP.  It uses
the PDPs that define time n*(300*288) to (n+1)*(300*288). Similar for
the other RRAs.

> Maybe this helps: the step for this rrd file is 300, but every minute a
> script collects data and update the rrd: executing 5 updates in five
> minutes results in 5 overwritten values? Even in this case, however, I
> don't understand how the MAX value is calculated.

If all is well, doing this shouldn't be different from just updating
once every 300 seconds.  A PDP is still generated using the available

MAX is just the maximum rate seen in any of the 288 PDPs that make up
one CDP.  Again, similar for the other RRAs.


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