Finally figured out what caused the problem: the LUN I owns the different storage processor than the one owned by the iSCSI endpoints which assigned to the AIX host when it's registered! The time when it was working for me, I chose a different raid group, which happened to be the same storage processor as the one for that iSCSI endpoint I chose, so it worked. In my later testings, I used another raid group (didn't realize that could cause the problem), and the same iSCSI endpoint, then didn't match, and kept getting failure.   Thank you so much for pointing me to "SP" and finally led to this result!!!!!
May I ask if I need to install any EMC components on AIX 5.3 in order to get access to CLARiiON iSCSI LUN? We have a AIX 5.3 server with iSCSI initiator configured and running properly, and provisioned a CLARiiON iSCSI LUN mapping to the AIX host. On the AIX server, after adding the CLARiiON target, and then running AIX "cfgmgr" and "lsdev -Cc disk" commands, we could see a new "hdisk10" shown up. However, when we tried to new a file system on this new device, or tried to create volume group on it, we kept getting the error below and couldn't access the disk at all.   # mkvg -f -y new_vg hdisk10 0516-1182 mkvg Open Failure on hdisk10. 0516-862 mkvg: Unable to create volume group.  Could this be because we don't have any EMC AIX components installed on the AIX server? If so, what AIX components we need to install? By googling around and saw someone mentioned "EMC.CLARiiON.aix.rte" and "EMC.CLARiiON.iscsi.rte". Do we have to install such packages in order to connect to a CLARiiON lun from AIX? Your help would be highly appreciated!!!  Regards,
AIX failed to open CLARiiON iSCSI LUN