Why logs shouldn't be put on RAID devices

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Here's your answer from Metalink note 38281.1 (Subject: RAID and Oracle - 20 Common Questions and Answers):

Placement of data files on RAID-5 devices is likely to give the best performance benefits, as these are usually accessed randomly. More benefits will be seen in situations where reads predominate over writes. Rollback segments and redo logs are accessed sequentially (usually for writes) and therefore are not suitable candidates for being placed on a RAID-5 device. Also, datafiles belonging to temporary tablespaces are not suitable for placement on a RAID-5 device.

Another reason redo logs should not be placed on RAID-5 devices is related to the type of caching (if any) being done by the RAID system. Given the critical nature of the contents of the redo logs, catastrophic loss of data could ensue if the contents of the cache were not written to disk, e.g. because of a power failure, when Oracle was notified they had been written. This is particularly true of write-back caching, where the write is regarded as having been written to disk when it has only been written to the cache. Write-through caching, where the write is only regarded as having completed when it has reached the disk, is much safer, but still not recommended for redo logs for the reason mentioned earlier.

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