Basic questions on rrd4j

I’ve been able to get persistence working with rrd4j and create a few graphs. (great!)
I have a few basic questions that don’t seem to be addressed.
I understand that this is a “round robin” database (new to me) but I don’t know how to find out how big the circle is. How many values will it store before it overwrites? Can this be changed?
I didn’t do anything to set up databases for the two energy monitors. They just work (nice) but I’d like to know how long they will store data before overwrite.

Did you see the documentation already?
As far as I remember, default is to persist the data for 365 days with different resolution on time axis.

I have just recently written a note for this thread addressing this topic.
Copied from there:

rrd4j does save the data in an adjustable number of archives, each one does hold the data backwards     from now for an adjustable timeframe. 
The first archive allways holds the data in a resolution of one entry per minute, each following archive is calculated from the entries in archive one by compressing a number of data points (using a “consdolidation-function” like: AVERAGE, MAX, MIN, etc).
    When creating a chart from a rrd4j database for a specific timeframe, the data is fetched from the archive that covers the desired timeframe.
    The setup of those archives is done in the file etc/openhab2/services/rrd4j.cfg, 
If such a file doesn’t exist openHAB will use a default setting

[consolidation function: AVERAGE, archives (covering time / resolution) 1 (8 hrs / 1 min), 2 (24 hrs / 4 min), 3 (150:16 hrs / 14 min), 4 (30 days / 1 hr), 5 ( 365 days / 12 hrs), 6 (3640 days / 7 days) ]

Coming back to the original question, yes you can change the setting of those archives (that would start the database completely new).
The size of each archive is set upfront ( never changes), each datapoint is moved stepwise backwards until it is “moved out” of the archive. When this end is reached depends on the setting for each archive.

Thanks for this additional information.
It does seem that a lot of what I was concerned about (size and aggregation) is taken care of automatically so maybe I should just not worry about it.

I’ll have to explore these options.