[rrd-users] Graphing

Christohper Pope cpope at sell.com
Thu Jan 3 19:42:19 MET 2002


MRTG/RRDTOOL always graphs in <something>/<some time>, which is normally just what I want to do,
but in some situations I want to graph just the number of times something happened during a time frame.
For example if I was keeping track of members logging into a site I would query 
(every 5 minutes) something like

SELECT COUNT(*) as count FROM users WHERE last_login > DATE_SUB( NOW(), INTERVAL 5 MINUTE );

Say this returns something like 35 users the first time, and 40 users the second time.

I issue an update into the users.rrd w/the following create definition:
rrdtool create --start `date +%s` user.rrd \
  	DS:signin:GAUGE:600:0:U \
   	RRA:AVERAGE:0.5:1:288 \
    	RRA:AVERAGE:0.5:3:1680 \
	RRA:AVERAGE:0.5:6:1440 \

Graphing this, the results are in signins per second and i get something like
(40 - 35)/(300 secs).  I want the scale to be in "per 5 minutes" so that what I
get is a graph of the number of users that signed in in that 5 minute interval,
and not the average. So i used a CDEF in my graph command
which should cancel with the division and leave me with (40 - 35) but instead
I get values of over 1000 and the site just isn't that popular.  Is there
something I'm missing?  

Chris Pope

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