direct IO or cache Io on LINUX server?

[mcclnx mcc]

we have several DELL 6800 with LSI 4e/DC adapter cards
(128 MB cache) in it.  DELL 6800 run under Redhar AS
4.0 and ORACLE 9ir2.  When I configured DELL 6800
server local disks (Mirror or RAID 5), it  have two
I/O policy to choice - direct IO and cache IO.

I doing simple test by copy files between differents
disk set and found  "direct IO" performance better
than "cache IO". It is against what I read before.  I
used to saw documents from SUN and IBM said "large
disk array cache will improve I/O performance".

Any one has suggestion FOR ORACLE server which I/O
policy should I setup?

Thanks.

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