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>][:daemon=
<address>]
VDEF:
vname=
RPN expression CDEF:
vname=
RPN expressionDESCRIPTION
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 is 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>][:daemon=
<address>]
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.
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.
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
DEF:ds0weekly=router.rrd:ds0:AVERAGE:daemon=collect1.example.com
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: "VDEF:avg=mydata,AVERAGE"
Note that currently only aggregation functions work in VDEF rpn
expressions. Patches to change this are welcome.
CDEF
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 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 <tobi@oetiker.ch>
This manual page by Alex van den Bogaerdt <alex@vandenbogaerdt.nl>
with corrections and/or additions by several people
1.8.0 2022-03-14 RRDGRAPH_DATA(1)
