The error is like follows:
scp /DB/VPROD/data01/sysaux01.dbf .
> cp: /DB/VPROD/data01/sysaux01.dbf: A system call received a parameter that is not valid.
This happens after we put database in hot backup mode and trying to copy datafiles.
So this approves what IBM told us, Oracle will use CIO no matter if the file system is mounted using CIO option. But this created another problem that AIX will not allow non-CIO system operation to access file opened with CIO.
There you go we back to square one, mounted the data volume back with CIO option in order to facilitate our Hot Backup. Another reason to use RMAN backup I guess.