=head1 NAME

rrdgraph_data - preparing data for graphing in rrdtool graph

=head1 SYNOPSIS

B<DEF:>I<E<lt>vnameE<gt>>=I<E<lt>rrdfileE<gt>>:I<E<lt>ds-nameE<gt>>:I<E<lt>CFE<gt>>[:step=I<E<lt>stepE<gt>>][:start=I<E<lt>timeE<gt>>][:end=I<E<lt>timeE<gt>>][:reduce=I<E<lt>B<CF>E<gt>>]

B<VDEF>:I<vname>=I<RPN expression>

B<CDEF>:I<vname>=I<RPN expression>

=head1 DESCRIPTION

These three instructions extract data values out of the B<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 B<RPN> which is described in its
own manual page.

Variable names (I<vname>) must be made up strings of the following characters
C<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
mixedcase names for variables since operators will always be in uppercase.

=head1 DEF

B<DEF:>I<E<lt>vnameE<gt>>=I<E<lt>rrdfileE<gt>>:I<E<lt>ds-nameE<gt>>:I<E<lt>CFE<gt>>[:step=I<E<lt>stepE<gt>>][:start=I<E<lt>timeE<gt>>][:end=I<E<lt>timeE<gt>>][:reduce=I<E<lt>B<CF>E<gt>>]

This command fetches data from an B<RRD> file.  The virtual name
I<vname> can then be used throughout the rest of the script. By
default, an B<RRA> which contains the correct consolidated data
at an appropriate resolution will be chosen.  The resolution can
be overridden with the L<--step|rrdgraph/item_Time> option.
The resolution can again be overridden by specifying the B<step size>.
The time span of this data is the same as for the graph by default,
you can override this by specifying B<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 B<CDP>
from an B<RRA>.  For instance, if your B<RRD> has an B<RRA> with
a resolution of 1800 seconds per B<CDP>, you should create an
image with width 400 and time span 400*1800 seconds (use appropriate
start and end times, such as C<--start end-8days8hours>).

If consolidation needs to be done, the B<CF> of the B<RRA> specified in the
B<DEF> itself will be used to reduce the data density. This behaviour can
be changed using C<:reduce=I<E<lt>B<CF>E<gt>>>.  This optional parameter
specifies the B<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

=head1 VDEF

B<VDEF>:I<vname>=I<RPN expression>

This command returns a value and/or a time according to the B<RPN>
statements used. The resulting I<vname> will, depending on the
functions used, have a value and a time component.  When you use
this I<vname> in another B<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: C<VDEF:avg=mydata,AVERAGE>

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

=head1 CDEF

B<CDEF>:I<vname>=I<RPN expression>

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

Example: C<CDEF:mydatabits=mydata,8,*>

=head1 About CDEF versus 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).

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
realy.

=head1 SEE ALSO

L<rrdgraph> gives an overview of how B<rrdtool graph> works.
L<rrdgraph_data> describes B<DEF>,B<CDEF> and B<VDEF> in detail.
L<rrdgraph_rpn> describes the B<RPN> language used in the B<?DEF> statements.
L<rrdgraph_graph> page describes all of the graph and print functions.

Make sure to read L<rrdgraph_examples> for tipsE<amp>tricks.

=head1 AUTHOR

Program by Tobias Oetiker E<lt>tobi@oetiker.chE<gt>

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