Hi Bronto:

I am not sure if i get it right if you are specifying something else than creating a data source (DS) using "RRDTool::OO". If not then my bad but If that is creating a COMPUTE DS then its pretty straight forward . COMPUTE data source is nothing but data source type ABSOLUTE. This is the same way i create a GAUGE data source using RRDTool::OO

#! /usr/bin/perl -w
#simple test ABSOLUTE DS
use strict;
use RRDTool::OO;

# creating new rrd object file 
  my $rrd = RRDTool::OO->new(
      file => "testrrd.rrd" );

# Create a round-robin database
    $rrd->create(
         step        => 1,  # one-second intervals
         data_source => { name      => "mynewDS",
                          type      => "ABSOLUTE" },
         archive     => { rows      => 5 });
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This would create a new rrd data file testrrd.rrd with DS the specified DS type.