I have 20 hard disks the are going to be deployed into multiple RAID 5 arrays for use with Oracle on an AIX host.
What is the optimal way to create/lay out the arrays?
I'm not an OCM or anything, but I can't imagine RAID 5 would be considered best practice. At my company we don't use RAID 5 for anything anymore because of the amount of times these arrays go bad with the large disks that have been coming to market. If you are using smaller SAS disks for example, you might be able to get away with it though, but I would say RAID 10 at the very least.
You may want to check out this link: http://blogs.zdnet.com/storage/?p=483 there are many others like it online as well talking about using RAID 10 instead of RAID 5. Again, at my company, when we roll out Oracle clients we will usually have large RAID 10 sets broken up out of 14-22 disks depending on what hardware we are using. The specific config for that I am not sure though because I am on the Windows side of things, where we also use RAID 10 almost exclusively.
A lot depends on the size and speed of the disks, if you're short on space then running the main database on RAID5 may be fine depending on your transaction volumes, but I'd recommend putting at least the log files as raw logical volumes on mirrored disks rather than RAID5.
However there's an awful lot of variables involved, from everything like the JFS2 mount settings (make sure it's set to concurrent I/O), and the disk queue lengths, to the RAID controller cache settings, so I can't give one simple answer.
Start with this IBM whitepaper, "Tuning IBM AIX 5.3 and AIX 6.1 for Oracle Database"
Stripe And Mirror Everything
You need to have at least the BASIC understanding of how to layout the drives for an Oracle system. Read the Oracle fundamentals documentation to understand how an Oracle uses it's memory, datafile's and CPU's.
You need to consider the relative performance / resilience for each of the important file groups: REDO, ARCHIVE LOGS, INDEXES, DATA, UNDO, OS, ORACLE_HOME
I'm no OCM either, but I am a certified Oracle DBA and EMC SAN administrator.
I reccomend avoiding raid 5 if possible, or at least only use it for archive logs and backup storage. At the very least, make sure the redo logs are on raid 1, raid 10 or no raid (multiplexed).
As the for generic layout, I reccomend having 3 seperate volumes allthough with most modern raid systems they may very well be on the same raid set - provided you are not utilizing parity.
Here is a typical layout:
the document linked in this answer points to SAME (stripe and mirror etc.) guide.
Looking at your question, I suppose the only other comment would be to ensure each mount/datastore is on a different channel.