[rrd-users] basic rrdtool usage - short intervals

[Charles Sprickman]

Hello,

I'm having some trouble with some very basic rrdtool setups.  I'm doing various 
benchmarking tasks and I want to sample some basic info while my benchmarks are 
running (load, disk tps, disk i/o, etc.).  I've got no problem collecting the 
data nor feeding it into rrdtool, but I think I'm missing something regarding 
the timing of pumping data into the db.  I'm trying to do very quick samples - 
once every 5 seconds or less so I can really see what's happening during the 
relatively short run of my benchmark.

What I'm finding when I dump the rrd is that there are a ton of empty values 
and the stepping is 3 minutes rather than the 15S I specified when I created 
the rrd:

1284268500: nan
1284268680: nan
1284268860: nan
1284269040: nan
1284269220: nan
1284269400: 1.6258215948e+03
1284269580: 1.6259667095e+03
1284269760: 1.6259498480e+03
1284269940: 1.6259465377e+03
1284270120: 1.6258741068e+03
1284270300: nan

The create command:

rrdtool create arcsize.rrd --start N --step 15 DS:arcsize:GAUGE:600:0:5000 
RRA:MIN:0.5:12:3660 RRA:MAX:0.5:12:3660 RRA:AVERAGE:0.5:12:3660

The update for this one in my script looks like this:

rrdtool update arcsize.rrd "N:$arcsize"

I have verified it's spitting out values; I print to STDOUT while it runs as 
well.  I run the script in a "while true" loop with a 14S sleep.  I've also 
tried manually updating with the same value, etc. with the same result that the 
steps in the dump of the rrd don't seem to match up.

I mostly used this guide to try and figure things out:
http://www.vandenbogaerdt.nl/rrdtool/tutorial/rrdtutorial.php

What am I missing?  I'm really hoping to get a basic working knowledge of 
rrdtool as I've been doing more and more stuff where it would be nice to graph 
arbitrary system stats at a high sample rate over a short period of time - this 
would certainly make the data easier to digest.

Thanks,

Charles