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Redo Log Metrics

These include:
Redo blocks written The number of redo database blocks written to the online redo logs.
Redo log buffer allocation entries The number of times space was allocated in the redo log buffer.
Redo entries The number of finite redo entries
Redo log space requests The number of times the log buffer requested additional space
Redo log space wait time The time spent waiting for redo log space
Redo size The size of the redo entries
Redo write time The amount of time spent writing to the online redo logs
Redo write latch time The time spent requesting redo write latches
Redo writes The number of physical redo write operations

Sizing Redo Log Files

The size of the redo log files can influence performance, because the behavior of the database writer and archiver processes depend on the redo log sizes. Generally, larger redo log files provide better performance. Undersized log files increase checkpoint activity and reduce performance.
Although the size of the redo log files does not affect LGWR performance, it can affect DBWR and checkpoint behavior. Checkpoint frequency is affected by several factors, including log file size and the setting of the FAST_START_MTTR_TARGET initialization parameter. If the FAST_START_MTTR_TARGET parameter is set to limit the instance recovery time, Oracle automatically tries to checkpoint as frequently as necessary. Under this condition, the size of the log files should be large enough to avoid additional checkpointing due to under sized log files. The optimal size can be obtained by querying the OPTIMAL_LOGFILE_SIZE column from the V$INSTANCE_RECOVERY view. You can also obtain sizing advice on the Redo Log Groups page of Oracle Enterprise Manager Database Control.
It may not always be possible to provide a specific size recommendation for redo log files, but redo log files in the range of a hundred megabytes to a few gigabytes are considered reasonable. Size your online redo log files according to the amount of redo your system generates. A rough guide is to switch logs at most once every twenty minutes.