# [rrd-users] Re: Question on rrdtool graph

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Mon Mar 19 23:35:04 MET 2001

```Chang-Ping Hsiao wrote:
> > Normally, you can fetch maximum values from the MAX RRA and average
> > values from the AVERAGE RRA.  Which CF to use is selected by DEF.
>
> 	Well, :-), this "all-AVERAGE" design is to the request from above,
> not up to me.  Since they don't need MAX or MIN, there are only AVERAGE's.

Then there is no problem as long as everybody understands that the
maximum of all averages can differ from the maximum of all maxima.
This result will be the same when you have an RRA with one PDP per
CDP ***and graph one CDP per pixel***, but it will differ when the
data is consolidated to have more than one PDP per CDP or pixel.

For instance:
- a data set with a resolution of 1 hour per CDP
- one value is 24, all others are 0 for the complete year,
- you graph 365 days on a graph with 365 pixels

This will result in a further consolidated data set covering one
day per pixel.  The average will therefore be *1* during one day
and 0 for all the others (we're talking UTC time here!). However,
the maximum of all averages will also be 1 whereas the maximum of
all maxima (if available and queried) would have been 24.
Big difference...

> > The purpose of having multiple RRAs is that you can store
> > data for a longer time, with a lower resolution.  There's
> > nothing agains having such a long "daily" RRA but it makes
> > no sense to have the "yearly" RRA with the same amount of
> > data.  It may speed up graphing a bit but I don't think
> > this is noticeable.
>
> 	For this point of view, I need to ask more if I may.  So if I want
> to have lower resolution for a year's worth of data, without saving them
> into RRA, how may I proceed and process?  I would love to learn how to save
> space and time for having the same result.  :-)

If you are graphing 400 days on a graph of 400 pixels, it doesn't
help you to have the data available at 5 minutes per CDP.  If you
want to display 400*5 minutes from data a year ago, it does help
to have the data available at 5 minutes per CDP.

To save on hard drive space at the expense of computing time:
If you know you need to graph ((one year ago)-1 day) to (one year ago)
then ditch the RRAs with lower resolution.  If you're only interested
in (one year ago) to (now) then ditch the bigger part of your high
resolution RRA.

To save on computing time at the expense of hard drive space:
Keep the data in many different resolutions.  Preferably the data
is available in every resolution you'll ever graph for every time
frame you'll ever graph.

> > Do, for instance:
> > rrdtool fetch --resolution 3600 --start '00:00' --end start+6h
> > and
> > rrdtool fetch --resolution 10800 --start '00:00' --end start+6h
>
> 	Thank you for this idea.  I never tried.  For some reason, the RRDs
> I have don't pop up information on these queries.  Maybe something is not
> right on them.  I'll check.

Perhaps you need to change the start to something else. Try "now-1d" or so.

cheers,
--
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