about poller_output and rrd_num

hi,all. i am using and reading source code on cacti-0.8.8a in windows xp,
i trace and find poller_output in database is first have data then was deleted,
so i enter into source code, i encountered to a problem, this occured in lib/poller.php,
i find process have went into the 433 line,
-----------------------------------------------
if ($k > 0) {
  db_execute("DELETE FROM poller_output WHERE local_data_id IN (" . implode(",", $data_ids) . ")");
       }
-----------------------------------------------
so i find the 419 line ----------------
if ($item["rrd_num"] <= sizeof($rrd_update_array{$item["rrd_path"]}["times"][$unix_time])) 
---------------------------------------
is executed,this lead to delete sql occured, i test cacti in 2012-11-05,
 and find rrd_num is always 1 and sizeof(...) is always 6,
i have questions about this is 
1,What is the meaning of rrd_num?
2,What is the meaning of sizeof($rrd_update_array{$item["rrd_path"]}["times"][$unix_time]))?
3,how can i make  poller_output have data  and not be deleted?
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