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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: