[rrd-users] RRA:AVERAGE - How does RRDtool know which RRA to use?

[A Darren Dunham]

On Wed, Aug 18, 2010 at 09:39:11AM -1000, Casey McGinty wrote:
> You specify the DS and archive function (AVERAGE). If there are multiple
> AVERAGEs then there is no way to specify which one RRDtool uses. Correct me
> if I am wrong.

You can "control" the choice to some extent by being very careful with
the time range of your graph and how that is covered by your data.

But you can also "specify" the RRA by providing a step value.

This came up just the other day.  In the rrdgraph man page:
$ man rrdgraph
[...]
       Time range
           [-s,--start time] [-e,--end time] [-S,--step seconds]
[...]
                               By default, rrdtool graph calculates the width
           of one pixel in the time domain and tries to get data from an RRA
           with that resolution.  With the step option you can alter this
           behaviour.  If you want rrdtool graph to get data at a one-hour
           resolution from the RRD, set step to 3600. Note: a step smaller
           than one pixel will silently be ignored.
-- 
Darren