One-off and no-data-problem - rrdtool

Question

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:
#!/bin/sh
RRD=test.rrd
STEP=300
HB=600
# 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 \
RRA:AVERAGE:0.5:1:1000
# Do a single update
rrdtool update $RRD \
3000000400:123
# 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 I've three questions:
Why does the fetch command start at 300, instead of 0?
Why does the fetch command include not only the last step (900) but also one more (1200)?
Why was not the updated value accepted?

Answer 1

The timeslot b contains information valid for b-step up to b EXCLUDING b itself. Hence when asking for data from 3000000000 to 3000000900 the first entry you get is 3000000300.
Since you are asking for data to end at 3000000900 you get the entry for 3000001200 as well as 3000000900 itself is the start of this entry.
At the moment even in gauge mode you would have to have a known value to start off ... so your first known update will simply bring you back into known state, it does not yet establish anything else. One might argue that in GAUGE mode this could be done differently.