[rrd-users] Difference between the values read and the values stored

David Knecht david.knecht at anyweb.ch
Tue Sep 23 13:32:51 MEST 2003

Hi all,

I'd like Cacti 0.8.3 and RRDtool 1.0.39 to store and graph ***exactly*** 
the value that it gets as its input.

I am using a GAUGE data type and the LAST consolidation function.

This is what the Cacti log is showing (thus, these are the values I'd 
like to see in the graphs):

09/22/2003 9:40 AM - CMD: /opt/rrdtool/bin/rrdtool update 
/var/www/html/cacti/rra/RTTSum-rtr_1.rrd --template RTTSum N:4726
09/22/2003 9:45 AM - CMD: /opt/rrdtool/bin/rrdtool update 
/var/www/html/cacti/rra/RTTSum-rtr_1.rrd --template RTTSum N:4102
09/22/2003 9:50 AM - CMD: /opt/rrdtool/bin/rrdtool update 
/var/www/html/cacti/rra/RTTSum-rtr_1.rrd --template RTTSum N:4891
09/22/2003 9:55 AM - CMD: /opt/rrdtool/bin/rrdtool update 
/var/www/html/cacti/rra/RTTSum-rtr_1.rrd --template RTTSum N:3855
09/22/2003 10:00 AM - CMD: /opt/rrdtool/bin/rrdtool update 
/var/www/html/cacti/rra/RTTSum-rtr_1.rrd --template RTTSum N:4724

And these are the values that are stored in the round robin database 
(output of the "rrdtool fetch" command):

1064216400: 4.7134000000e+03
1064216700: 4.1144800000e+03
1064217000: 4.8725900000e+03
1064217300: 3.8722666667e+03
1064217600: 4.7066200000e+03
1064217900: nan

As you can see there is a slight difference between what Cacti reports 
and what rrdtool stores/graphs (in fact, each value differs towards its 
preceding value).

I quote Alex van den Bogaerdt's RRDtool tutorial at


"We can tell RRDtool to store the values we measure directly as they are 
(this is not entirely true but close enough)."

Does this mean that RRDTool does not/cannot store values in its raw 
form, even if using GAUGE data types? Is it correct that RRDTool is 
doing some time compensation even for GAUGE data types (which to my 
latest understanding are not related to time?) Might any other averaging 
mechanism fool me here?

Thanks, David

PS: I have raised this issue in the Cacti forum before and am now 
switching over here.

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