[rrd-users] Re: trouble with start time of RRD

[Alex van den Bogaerdt]

Matthew F. Ringel wrote:

> I set up an RRD with the command:
> 
> rrdtool create big.rrd --start=965088000 --step=300 \
> DS:inbytes:COUNTER:600:U:U DS:outbytes:COUNTER:600:U:U \
> RRA:AVERAGE:.5:1:8640
> 
> Where 965088000 is midnight of 1 August 2000.
> 
> What perplexes me is that doing a dump of big.rrd shows the following:
> 
> [...]
>    <lastupdate> 965088000 </lastupdate> <!-- 2000-08-01 00:00:00 GMT -->
> [...]
>    <database>
>    <!-- 2000-07-02 00:05:00 GMT --> <row><v> NaN </v><v> NaN </v></row>
>    <!-- 2000-07-02 00:10:00 GMT --> <row><v> NaN </v><v> NaN </v></row>
>    <!-- 2000-07-02 00:15:00 GMT --> <row><v> NaN </v><v> NaN </v></row>
> [...]
>    <!-- 2000-08-01 00:00:00 GMT --> <row><v> NaN </v><v> NaN </v></row>
>    </database>
> 
> It appears that the *end* time (and *not* the start time) for the RRD
> is the date I specified with the --start option.  Am I missing
> something?  I could do some date voodoo to compensate for this, but
> I'd prefer not to.  I just want the first entry in the RRD to be at
> least near the time I specify in the --start option.

No you don't.

Suppose that the first entry in the RRD would be august 1st.
The database would then have to know about august 1st until
(august 1st) + (8640*300 seconds) == 967680000 == aug 31st 00:00

You can't "refresh" the entries in the database and therefore you can't
update aug 1 .. aug 31.

The way it is now:
the database knows about jul 2 .. aug 1 00:00.  The first update that
should be made is aug 1 00:05 which probably is what you want. It will
overwrite the oldest slot in the database --> 2000-07-02 00:05:00 GMT

Two side notes:
1) start the database a few seconds before aug 1 00:00 so you can enter
   something in it for aug 1 00:00
2) keep more records.  You now have 8640 slots of 5 second each.  That
   is 8640*300/86400 == 30 days exact.  This is *not* a month.

regards,
-- 
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