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Guidelines for Sizing Redo Logs for an Oracle Database

This topic is part of Configuring an Oracle Database for Siebel Applications.

If redo logs are too small, frequent log switches occur, creating resource-intensive Oracle Database check-pointing in which all dirty buffers are flushed. A range of 10 to 15 minutes or longer for log switching is preferable under a normal OLTP (Online Transaction Processing) load. However, during periods of heavy DML (data manipulation language) activity (for example, during large EIM loads or upgrades), the logs may switch more frequently than every two minutes. When this occurs, overall database performance will suffer as a result.

You can check the frequency of this operation either in the alert log or by querying v$loghist. It is best to use verification when there is the greatest activity and the heaviest load on the database.

If this activity occurs too frequently, drop and re-create individual redo log groups with larger sizes. A suggested minimum size is 300 MB.

To achieve optimum performance, placing subsequent log file groups on alternative devices is critical. This precaution prevents the archiver process (ARCH) and the log writer process (LGWR) from competing for I/Os on the same device, because ARCH reads from the previous group, while LGWR is writing to the current group. This process causes the read-write head of the device to move back and forth, contributing to inefficient I/O. When log file groups are located on separate devices, the speed of both processes improves as they do not contend for the same hardware resource.