rrdtool

How to understand Primary Data Point(PDP) in rrdtool database?

If I dump an RRD to XML, then under "PDP Status" section there are three elements: <last_ds>, <value> and <unknown_sec>. For example:

            <!-- PDP Status -->
            <last_ds>90</last_ds>
            <value>4.2177496500e+03</value>
            <unknown_sec> 184 </unknown_sec>

Now as I understand, then each time I execute "rrd update", I will update Primary Data Point (PDP). Looks like whatever I put as a value for rrdtool update(for example rrdtool update test.rrd "N:abc"), then it is shown as a value for <last_ds> element. However, how is the number for <value> calculated? I mean the number 4217.7496500 in example above. Is this some kind of average? Last but not least, while I understand that <unknown_sec> shows the number of seconds when value of the DS has been unknown, then this counter seems to wrap around 280 - 295 seconds. How to explain this? I mean for example if I execute while true; do rrdtool update test.rrd "N:75"; rrdtool dump test.rrd | grep "<unknown_sec>"; sleep 1; done where 75 is lower than the lowest value allowed for this DS, then output is following:

   /* data not shown for brevity */
   <unknown_sec> 280 </unknown_sec>
   <unknown_sec> 281 </unknown_sec>
   <unknown_sec> 282 </unknown_sec>
   <unknown_sec> 0 </unknown_sec>
   <unknown_sec> 1 </unknown_sec>
   <unknown_sec> 2 </unknown_sec>
   /* data not shown for brevity */
Solution

  • the PDP content of <value> is the sum of all products of input value multiplied by duration this value was valid for. In order to build the PDP, at the end of the interval, this value is divided by the duration of the interval minus the number of unknown seconds ... The number of unknown seconds resets to 0 when a new interval is started ...