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If you have root filling up and see a bogus file in /dev:

-rw-r–r–    1 root     system         861 Sep 27 19:14 null 2>&1

Then you are having the following issue with the Systems Director Common Agent (cas agent).  reference:

Director Agent 6.2.1 on AIX Might Fill Up / Filesystem

http://www-304.ibm.com/support/docview.wss?uid=nas74d33539b559cc0308625792900533a8f

Also, here’s good write up on how to check why root file system is full:

/ (root) overflow

http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.baseadmn/doc/baseadmndita/fsrootover.htm