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man pages section 1: User Commands

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Updated: Wednesday, February 10, 2021

rrdgraph_data (1)


rrdgraph_data - preparing data for graphing in rrdtool graph



VDEF:vname=RPN expression

CDEF:vname=RPN expression


RRDGRAPH_DATA(1)                    rrdtool                   RRDGRAPH_DATA(1)

       rrdgraph_data - preparing data for graphing in rrdtool graph


       VDEF:vname=RPN expression

       CDEF:vname=RPN expression

       These three instructions extract data values out of the RRD files,
       optionally altering them (think, for example, of a bytes to bits
       conversion). If so desired, you can also define variables containing
       useful information such as maximum, minimum etcetera. Two of the
       instructions use a language called RPN which is described in its own
       manual page.

       Variable names (vname) must be made up strings of the following
       characters "A-Z, a-z, 0-9, _, -" and a maximum length of 255

       When picking variable names, make sure you do not choose a name that is
       already taken by an RPN operator. A safe bet it to use lowercase or
       mixed case names for variables since operators will always be in


       This command fetches data from an RRD file.  The virtual name vname can
       then be used throughout the rest of the script. By default, an RRA
       which contains the correct consolidated data at an appropriate
       resolution will be chosen.  The resolution can be overridden with the
       --step option.  The resolution can again be overridden by specifying
       the step size.  The time span of this data is the same as for the graph
       by default, you can override this by specifying start and end.
       Remember to escape colons in the time specification!

       If the resolution of the data is higher than the resolution of the
       graph, the data will be further consolidated. This may result in a
       graph that spans slightly more time than requested.  Ideally each point
       in the graph should correspond with one CDP from an RRA.  For instance,
       if your RRD has an RRA with a resolution of 1800 seconds per CDP, you
       should create an image with width 400 and time span 400*1800 seconds
       (use appropriate start and end times, such as "--start

       If consolidation needs to be done, the CF of the RRA specified in the
       DEF itself will be used to reduce the data density. This behavior can
       be changed using ":reduce=<CF>".  This optional parameter specifies the
       CF to use during the data reduction phase.

       It is possible to request single data sources from a specific
       RRDCacheD, see rrdcached, using the ":daemon=<address>" parameter. The
       value given to this parameter follows the same syntax as other means to
       specify the address of the caching daemon. It is described in detail in
       rrdcached. Beware, however, that colons (in IPv6 addresses and as a
       port separator, for example) need to be escaped using a backslash.



       VDEF:vname=RPN expression

       This command returns a value and/or a time according to the RPN
       statements used. The resulting vname will, depending on the functions
       used, have a value and a time component.  When you use this vname in
       another RPN expression, you are effectively inserting its value just as
       if you had put a number at that place.  The variable can also be used
       in the various graph and print elements.

       Example: "VDEF:avg=mydata,AVERAGE"

       Note that currently only aggregation functions work in VDEF rpn
       expressions.  Patches to change this are welcome.

       CDEF:vname=RPN expression

       This command creates a new set of data points (in memory only, not in
       the RRD file) out of one or more other data series. The RPN
       instructions are used to evaluate a mathematical function on each data
       point. The resulting vname can then be used further on in the script,
       just as if it were generated by a DEF instruction.

       Example: "CDEF:mydatabits=mydata,8,*"

About CDEF versus VDEF
       At some point in processing, RRDtool has gathered an array of rates
       ready to display.

       CDEF works on such an array.  For example, 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.

       VDEF also works on such an array but in a different way.  For example,
       VDEF:max=ds0,MAXIMUM would scan each of the array members and store the
       maximum value.

   When do you use VDEF versus CDEF?
       Use CDEF to transform your data prior to graphing.  In the above
       example, we'd use a CDEF to transform bytes to bits before graphing the

       You use a VDEF if you want 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).

       If you want to apply 'complex' operations to the result of a VDEF you
       have to use a CDEF again since VDEFs only look like RPN expressions,
       they aren't really.

       See attributes(7) for descriptions of the following attributes:

       |Availability   | image/rrdtool    |
       |Stability      | Uncommitted      |
       rrdgraph gives an overview of how rrdtool graph works.  rrdgraph_data
       describes DEF,CDEF and VDEF in detail.  rrdgraph_rpn describes the RPN
       language used in the ?DEF statements.  rrdgraph_graph page describes
       all of the graph and print functions.

       Make sure to read rrdgraph_examples for tips&tricks.

       Program by Tobias Oetiker <tobi@oetiker.ch>

       This manual page by Alex van den Bogaerdt <alex@vandenbogaerdt.nl> with
       corrections and/or additions by several people

       This software was built from source available at
       https://github.com/oracle/solaris-userland.  The original community
       source was downloaded from

       Further information about this software can be found on the open source
       community website at http://oss.oetiker.ch/rrdtool/.

1.5.999                           2016-02-18                  RRDGRAPH_DATA(1)