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.
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
././rrdgraph.html#item_time
--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 behaviour 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:
VDEF:avg=mydata,AVERAGE
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,*
SEE ALSO
././rrdgraph.html
the rrdgraph manpage
gives an overview of how
rrdtool graph
works.
././rrdgraph_data.html
the rrdgraph_data manpage
describes
DEF
,
CDEF
and
VDEF
in detail.
././rrdgraph_rpn.html
the rrdgraph_rpn manpage
describes the
RPN
language used in the
?DEF
statements.
././rrdgraph_graph.html
the rrdgraph_graph manpage
page describes all of the graph and print functions.
Make sure to read
././rrdgraph_examples.html
the rrdgraph_examples manpage
for tips&tricks.
AUTHOR
Program by Tobias Oetiker <
mailto:oetiker@ee.ethz.ch
oetiker@ee.ethz.ch
>
This manual page by Alex van den Bogaerdt <
mailto:alex@ergens.op.het.net
alex@ergens.op.het.net
>
