[rrd-users] Multiple RRA for pdp_per_row of 1

Steve Shipway s.shipway at auckland.ac.nz
Wed Dec 31 00:07:52 CET 2008

> > I knew there was something I was missing.  If you try to graph MAX and
> > only have AVERAGE rra for pdp_per_row it will use the first MAX rra it
> > finds instead.
> >
> > Is there a way around this without forcing the resolution or do I need
> > to store the MAX rra and just fill it with the AVERAGE data?
> Any ideas?   I am guessing I'm going to be forced to duplicate my AVERAGE
> rra into a MAX rra for pdp_per_row 1.

Since, at pdp_per_row=1, MAX is effectively the same as AVG, your MAX RRA
would be redundant except that it would, as you say, prevent a higher
pdp_per_row MAX RRA from being taken in preference if you ask for a MAX at
this resolution.  However, why would you be asking for a MAX at this
resolution?  If you're coding this yourself then just take the AVG instead
as it is the same data - this is why MRTG doesn't display peak lines for
'Daily' graphs.

On reflection, though, I notice that MRTG DOES create a pdp_per_row=1 MAX
RRA by default as I think it helps with other time-period MAX calculations -
if you ask for a single value of max(ds) between t0 and t1, I believe it
looks for a MAX RRA with the least overlap from (t0,t1] to calculate from.
This may be a candidate for RRDtool optimisation (if you need a
pdp_per_row=1 MAX RRA and one is not available, then a pdp_per_row=1 AVG or
LAST will do just as well)

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