RRD Size Estimates
Here are some various descriptions of storage granularity, along with the estimated size of the RRD (on disk):
hourly, for seven days
six-hourly for two weeks
daily for 2 months
weekly for 6 months
monthly(4 weeks) for 1 year
quarterly for 2 years
yearly for 6 years
total:
2 stats - 24kB
3 stats - 35kB
4 stats - 46kB
hourly for 29 days
6-hourly for 175 days
daily for two years
weekly for thirteen years
monthly for 58 years
quarterly for 175
yearly for 700 years
total:
2 stats - 316kB
3 stats - 474kB
4 stats - 631kB
Both of these measurements are storing data from two data sources (nominally the user and project activity statistics), and keeping archives of the average, min, max, and total for all the periods listed above. Adding another data source causes the second archive to go from 316kB to 474kB. Adding yet another data source makes it jump to 631kB. This indicates a per-data source cost of about 157kB, when you go to unreasonably long retention periods.