[rrd-users] One-off and no-data-problem

Jonatan Magnusson jonatan at cmteknik.se
Thu Dec 30 11:28:56 CET 2010


I'm just getting started using RRDtool to collect climate data. I don't 
use the graph functionality, but rather use "fetch" to retrieve data. I 
then use another graphing solution (flot) to display the data, and that 
seems to work somewhat. But I had some small problems and decided to 
check the details of the update and fetching and was suddenly not so 
sure that things worked as I expected.

So I've created a tiny shell script that creates a database, put a 
single value in it and then print the contents:



     # Remove previous databse to be sure that
     # old data does not affect the test
     rm -f $RRD

     # Create database
     rrdtool create $RRD \
         --start 2999999999 --step $STEP \
         DS:a:GAUGE:$HB:U:U \

     # Do a single update
     rrdtool update $RRD \

     # Fetch data and print to stdout
     rrdtool fetch $RRD \
         --start 3000000000 --end 3000000900 AVERAGE

I would expect this to print three (or perhaps four, not sure about the 
last one) values like this:

     3000000000: -nan
     3000000300: 123
     3000000600: -nan
     3000000900: -nan

But this is what I get:

     3000000300: -nan
     3000000600: -nan
     3000000900: -nan
     3000001200: -nan

So there's three differences:

1. Why does the fetch command start at 300, instead of 0?
2. Why does the fetch command include not only the last step (900) but 
also one more (1200)?
3. Why was not the updated value accepted?

(Actually, my initial problems was another thing, but I must understand 
this before I can be sure about the rest.)


Jonatan Magnusson

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