Without diving deep into this problem, the first thing that pops up in my mind is:
Do you know about, and understand, data normalization and consolidation? http://rrdtool.vandenbogaerdt.nl/process.php Be aware that consolidation-like behaviour may also occur while graphing, so the number of horizontal pixels vs. the number of time slots does affect the outcome. cheers, Alex ----- Original Message ----- From: "Wesley PA4WDH" <[email protected]> To: <[email protected]> Sent: Sunday, February 17, 2013 11:13 AM Subject: [rrd-users] Strange behavior when using CDEF to divide two values > Hi All, > > I'm running some rrdtool data collection and i see some strange behavior > when i try to divide two DS's with a CDEF. > > I'm collecting the length of traffic jams and the number of traffic jams > in my country and store them in a rrd database. All is well and graphs > look good. Now i would like to calculate the average length by dividing > the total length with the number. I thought this would be easy by just > writing a CDEF:AVG=LENGTH,COUNT,/ line and graphing AVG ... but how wrong > could i be .. :-) > > First i got a lot of high spikes in my graph. I noticed this happened when > the "COUNT" was 0, so actually a division by zero. I thought this would > render to NaN but it didn't :-) I solved this by storing NaN in the rrd > when the value is 0. Are there any other/better solutions ? > > Now the graph looks nice but the value isn't correct. I saved some files > for you to see: http://ernstagn.home.xs4all.nl/rrd/ > Here you see the three graphs (length number and average) and dump.xml is > a "rrdtool dump" xml of the database. > You can see that the value plotted for the average is not correct given > the input values. For example at 11:00 at the left of the graph: Total > length was 12km, and the number was 4, so the average should be 3. > However, the average graph plots at about 1.7. > During midnight there is a similar example: There are 4 traffic jams of > about 8km, but the average hardly plots above 0. > I've tried with "length" and "count" swapped in the RPN, but that didn't > work out either. > > Just to make sure i'm not making any mistake with the calculation i tried > to make an awk script which does the same with an xml export and there i > get the values i'd expect. > > Does anyone know how to solve this ? I'm using rrdtool version 1.4.7. _______________________________________________ rrd-users mailing list [email protected] https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users
