[rrd-users] rrdtool getting incorrect values after update

spalxy spalxy at gmail.com
Wed Mar 13 14:27:01 CET 2013

Hi guys, i have been searching over the docs a lot the last few day, hence i
am very new to RRD and i've been trying to get something going. I am trying
to migrate from an old software of mine, which uses RRD to a new software,
which doesnt. So far i've figured out how to dump the rrd tables and what
does rrd to when saving the values, however i've had issues getting the same
values that get stored inside rrd. Here is a quick example i made, hope you
can give me a hand.

I made a simple php file, which runs every 300 seconds and updates a RRD
database. The cycle checks if the .rrd file exists and if it doest, then it
creates it:
Here is my create statement (the one my old software uses): 
rrdtool create test.rrd DS:cpu:DERIVE:800:0:112500 RRA:AVERAGE:0.5:1:600

And immediately after the file is created, the script runs an update
rrdtool update test.rrd N:5848

(the value above is an example). Now my script updates exactly every 300
seconds after it has started running, so i'm sure the timing is exactly 300
seconds apart. Now, here are the values i have tested with: (timestamp -
1363128441 - 5791
1363128741 - 6665
1363129041 - 7482

And here are the results rrd has stored (this is from a xml dump):
 <row><v> 2.9050692504e+00 </v></row>
 <row><v> 2.8128932327e+00 </v></row>

Now as per my understanding, the first value should be the result from: 
(6665 - 5791)/300 = 2,913333333333333 ... which doesnt match the first
value. The same issue applies to the second value and any other value i have
tested. What am i doing wrong ? 
Thank you.

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