Editing RRDtool (section)

==General data storage==
RRDtool assumes time-variable data in intervals of a certain length. This interval, usually named '''step''', is specified upon creation of an RRD file and cannot be changed afterwards. Because data may not always be available at just the right time, RRDtool will automatically interpolate any submitted data to fit its internal time-steps.

The value for a specific step, that has been interpolated, is named a primary data point ('''PDP'''). Multiple PDPs may be consolidated according to a consolidation function ('''CF''') to form a consolidated data point ('''CDP'''). Typical consolidation functions are [[average]], minimum, maximum.

After the data has been consolidated, the resulting CDP is stored in a round-robin archive ('''RRA'''). A round-robin archive stores a fixed number of CDPs and specifies how many PDPs should be consolidated into one CDP and which CF to use. The total time covered by an RRA can be calculated as follows:

: time covered = (#CDPs stored) × (#PDPs per CDP) × (step time length)

After this time the archive will "wrap around": the next insertion will overwrite the oldest entry. This behavior in this context is referred to as "round-robin" and is the reason for the program's name.  However this is different from the common computer science definition, which is a method of distributing resources among multiple consumers or processes.

To cover several timespans and/or use several consolidation functions, an RRD file may contain multiple RRAs. The data retrieval function of RRDtool automatically selects the archive with the highest resolution that still covers the requested timespan. This mechanism is also used by RRDtool's graphing subsystem.