Hi Alex, At least i thought i did understand :-) I also see that the values stored in the RRD databases are not the exact values i store in it, as far as i know a result of normalisation.
The part i do not understand is that it may occour during graphing. The two datasources are stored in the same database and normalized so i wouldn't expect any normalization during graphing. I don't think consolidation will have any role in this situation since i'm only using one RRA to store a day of 5-minute samples, not the data is not consolidated any further. Please correct me if i'm wrong :-) Best regards, Wesley --- On Sun, 2/17/13, Alex van den Bogaerdt <[email protected]> wrote: > From: Alex van den Bogaerdt <[email protected]> > Subject: Re: [rrd-users] Strange behavior when using CDEF to divide two values > To: [email protected] > Date: Sunday, February 17, 2013, 11:57 AM > 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 > _______________________________________________ rrd-users mailing list [email protected] https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users
