RRDGRAPH_DATA(1) rrdtool RRDGRAPH_DATA(1)
NAME
rrdgraph_data - preparing data for graphing in rrdtool graph
SYNOPSIS
DEF:<vname>=<rrdfile>:<ds-name>:<CF>[:step=<step>][:start=<time>][:end=<time>][:reduce=<CF>]
VDEF:vname=RPN expression
CDEF:vname=RPN expression
DESCRIPTION
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 characters.
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 uppercase.
DEF
DEF:<vname>=<rrdfile>:<ds-name>:<CF>[:step=<step>][:start=<time>][:end=<time>][:reduce=<CF>]
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 end-8days8hours").
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.
Example:
DEF:ds0=router.rrd:ds0:AVERAGE
DEF:ds0weekly=router.rrd:ds0:AVERAGE:step=7200
DEF:ds0weekly=router.rrd:ds0:AVERAGE:start=end-1h
DEF:ds0weekly=router.rrd:ds0:AVERAGE:start=11\:00:end=start+1h
VDEF
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: "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 bits.
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 ALSO
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.
AUTHOR
Program by Tobias Oetiker <[email protected]>
This manual page by Alex van den Bogaerdt <[email protected]> with corrections and/or additions by
several people
1.4.8 2013-05-23 RRDGRAPH_DATA(1)
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