CDPs PDPs RRAs RRDTools xff

  RRA:CF:cf arguments
       The purpose of an RRD is to store data in the round robin archives (RRA). An archive consists of a number of
       data values or statistics for each of the defined data-sources (DS) and is defined with an RRA line.

       When data is entered into an RRD, it is first fit into time slots of the length defined with the -s option,
       thus becoming a primary data point.

       The data is also processed with the consolidation function (CF) of the archive. There are several consolidation
       functions that consolidate primary data points via an aggregate function: AVERAGE, MIN, MAX, LAST.

       AVERAGE
           the average of the data points is stored.

       MIN the smallest of the data points is stored.

       MAX the largest of the data points is stored.

       LAST
           the last data points is used.

       Note that data aggregation inevitably leads to loss of precision and information. The trick is to pick the
       aggregate function such that the interesting properties of your data is kept across the aggregation process.

       The format of RRA line for these consolidation functions is:

       RRA:AVERAGE | MIN | MAX | LAST:xff:steps:rows

       xff The xfiles factor defines what part of a consolidation interval may be made up from *UNKNOWN* data while
       the consolidated value is still regarded as known. It is given as the ratio of allowed *UNKNOWN* PDPs to the
       number of PDPs in the interval. Thus, it ranges from 0 to 1 (exclusive).

       steps defines how many of these primary data points are used to build a consolidated data point which then goes
       into the archive.

       rows defines how many generations of data values are kept in an RRA.  Obviously, this has to be greater than
       zero.

      rrdtool create target.rrd \
                --start 1023654125 \
                --step 300 \
                DS:mem:GAUGE:600:0:671744 \
                RRA:AVERAGE:0.5:12:24 \
                RRA:AVERAGE:0.5:288:31

       The next line declares a round robin archive (RRA). The syntax for declaring an RRA is

        RRA:CF:xff:step:rows

       RRA is the keyword to declare RRAs. The consolidation function (CF) can be AVERAGE, MINIMUM, MAXIMUM, and LAST.
       The concept of the consolidated data point (CDP) comes into the picture here. A CDP is CFed (averaged,
       maximum/minimum value or last value) from step number of PDPs. This RRA will hold rows CDPs.

       Lets have a look at the example above. For the first RRA, 12 (steps) PDPs (DS variables) are AVERAGEed (CF) to
       form one CDP. 24 (rows) of theses CDPs are archived. Each PDP occurs at 300 seconds. 12 PDPs represent 12 times
       300 seconds which is 1 hour. It means 1 CDP (which is equal to 12 PDPs) represents data worth 1 hour. 24 such
       CDPs represent 1 day (1 hour times 24 CDPs). This means, this RRA is an archive for one day. After 24 CDPs, CDP
       number 25 will replace the 1st CDP. The second RRA saves 31 CDPs; each CPD represents an AVERAGE value for a
       day (288 PDPs, each covering 300 seconds = 24 hours). Therefore this RRA is an archive for one month. A single
       database can have many RRAs. If there are multiple DSs, each individual RRA will save data for all the DSs in
       the database. For example, if a database has 3 DSs and daily, weekly, monthly, and yearly RRAs are declared,
       then each RRA will hold data from all 3 data sources.