|Disks for redo devices|
When a transaction is committed, the redo log entry in the redo log buffer must be written to disk. The characteristics of the redo log writes are very different from that of the data file I/O. The differences include:
- They are sequential I/Os. Each access follows the previous access on the disk. The disk drive does not have to seek for the disk block to access. This means sequential I/Os are much faster than random I/Os. Most disk devices can perform about one hundred sequential I/Os per second.
- The I/Os are write-only and attempt to write through any disk cache.
These factors combined suggest that transaction processing is optimized if a redo log is on a dedicated disk device. If your database is primarily read-only, redo log I/O is unlikely to be an issue, and the redo logs can be placed in (virtually) any convenient location.