This type of question is becoming more frequent, especially as hardware vendors sell larger and larger disk volumes.
First off, be wary of using RAID 5 for an Oracle database. RAID levels 3 and 5 employ a "write-penalty". It takes longer to write to RAID 5 volumes than it does to write to non-RAID volumes. This is because parity bits need to be computed and stored as well as storing your data. This can slow things down considerably for write-intensive database operations.
Some people will tell you to never use RAID 5 for an Oracle database. I have a database here where I am using RAID 5, but that data is read-mostly. Things I *never* put on RAID 5 are control files, online redo logs, temp tablespace, and the archive log destination. These objects are write intensive and RAID 5 will slow down database operations if these objects are on RAID 5 devices.
Now on to your question... One can just throw datafiles on a RAID 5 disk volume. Yes, RAID will take care of striping those datafiles. So your chances of an index and a table falling on the same volume are decreased. But those chances are not eliminated. You have no control over where the index and table will fall. To maintain that control, I suggest that you use multiple RAID 5 partitions. This way, you can be guaranteed of separating multi-concurrent objects (such as a table and its index).
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This was first published in February 2002