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.
+
=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>>]
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.
+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
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>).
-=for comment
-XXX is the above start/end syntax correct? Fritz
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
Example: C<VDEF:avg=mydata,AVERAGE>
+Note that currently only agregation functions work in VDEF rpn expressions.
+Patches to change this are welcome.
+
=head1 CDEF
B<CDEF>:I<vname>=I<RPN expression>