[ I originally sent this directly to Tobi and Alex. Given their e-mail addresses are published, I suspect they must get a jillion spam messages a day. I'm posting this here. ]
Hi, I'm using RRD and did some reserch to determine the difference between CDEV and VDEF. I came across the following link: [http://osdir.com/ml/db.rrdtool.devel/2006-03/msg00001.html] I followed the reply-to's and found the original poster didn't follow up with the request to update the .pod I found Alex's information clear and mostly took it and placed it in the attached .diff I made very minor modifications. Please review and let me know if there are any changes you'd like me to roll in. Thank you for such an awesome tool; the submission is the least I can do to say thank you. :) -- Pablo Sanchez - Blueoak Database Engineering, Inc Ph: 819.459.1926 Fax: 760.860.5225 (US)
Index: rrdgraph_data.pod =================================================================== --- rrdgraph_data.pod (revision 1796) +++ rrdgraph_data.pod (working copy) @@ -10,6 +10,8 @@ B<CDEF>:I<vname>=I<RPN expression> +B<CDEV> versus B<VDEF> + =head1 DESCRIPTION These three instructions extract data values out of the B<RRD> files, @@ -90,6 +92,33 @@ Example: C<CDEF:mydatabits=mydata,8,*> +=head1 CDEF versus VDEF + +This section attempts to clarify the difference between B<CDEF> and +B<VDEF>. + +At some point in processing, B<RRDtool> has gathered an array of rates +ready to display. + +B<CDEF> works on such an array. For example, I<CDEF:new=ds0,8,*> +would multiply each of the array members by eight (probably +transforming bytes into bits). The result is an array containing the +new values. + +B<VDEF> also works on such an array but in a different way. For +example, I<VDEF:max=ds0,MAXIMUM> would scan each of the array members +and store the maximum value. + +=head2 When do you use B<VDEF> versus B<CDEF>? + +Use B<CDEF> to transform your data prior to graphing. In the above +example, we'd use a B<CDEF> to transform bytes to bits before +graphing the bits. + +You use a B<VDEF> if you want I<max(1,5,3,2,4)> to return five which +would be displayed in the graph's legend (to answer, what was the +maximum value during the graph period). + =head1 SEE ALSO L<rrdgraph> gives an overview of how B<rrdtool graph> works.
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