reset rrd_state for grapv in ruby bindings -- Sven Engelhardt

[rrdtool.git] / doc / rrdgraph_data.pod

diff --git a/doc/rrdgraph_data.pod b/doc/rrdgraph_data.pod
index 9e37efd..d5ceddb 100644 (file)
--- a/doc/rrdgraph_data.pod
+++ b/doc/rrdgraph_data.pod
@@ -1,6 +1,6 @@
 =head1 NAME
 
-rrdgraph_data - Preparing data for graphing
+rrdgraph_data - preparing data for graphing in rrdtool graph
 
 =head1 SYNOPSIS
 
@@ -12,11 +12,19 @@ B<CDEF>:I<vname>=I<RPN expression>
 
 =head1 DESCRIPTION
 
-These three instructions collect the data out of the B<RRD> files,
-optionally altering it (think of a bytes to bits conversion) and
-if desired you can 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.
+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
+mixed case names for variables since operators will always be in uppercase.
 
 =head1 DEF
 
@@ -26,14 +34,14 @@ 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. 
+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 off
-graph, the data will be further consolidated.  This may result in
+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
@@ -42,7 +50,7 @@ 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
+B<DEF> itself will be used to reduce the data density. This behavior 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.
 
@@ -67,6 +75,9 @@ 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>
@@ -79,17 +90,46 @@ 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
+really.
+
 =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_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>oetiker@ee.ethz.chE<gt>
+Program by Tobias Oetiker E<lt>tobi@oetiker.chE<gt>
 
-This manual page by Alex van den Bogaerdt E<lt>alex@ergens.op.het.netE<gt>
+This manual page by Alex van den Bogaerdt E<lt>alex@vandenbogaerdt.nlE<gt>
+with corrections and/or additions by several people