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Oracle Tips by Burleson

Oracle9i - Database Managed Undo Segments

• Although the test covers Oracle9i, candidates should also have a firm understanding of manual undo (rollback) segments
   

• You must know the contents of v$undostat and how it is used to size undo segment tablespaces

Redo Logs

On-line and archived redo logs are the base components of the Oracle instance and roll-forward recovery processes. Redo logs must also be sized correctly to obtain optimal performance.

Checkpoint Not Complete

Checkpoints ensure that all modified data buffers are written to the database files. One (out of several) reasons Oracle can fire a checkpoint is when it switches from one log group to another. When Oracle fills a log group, a checkpoint is fired and Oracle then begins writing to the next log group. This continues in a circular fashion; when all log groups are filled Oracle reuses the first group. The process by which DBWR  writes modified data buffers to disk in Oracle is not synchronized with the COMMIT of the corresponding transactions. The checkpoint ensures that all the modified data buffers in the cache that are covered by the current log are written to the corresponding data files.

A common rule of thumb is to adjust the redo log’s size so that Oracle performs a log switch every 15 to 30 minutes. Log switches that occur more frequently may have a negative impact on performance. Log switches that occur several times a minute have a definite impact on database performance. Checking messages in the alert log is an easy way to determine how fast Oracle is filling and switching logs. If the following messages are found, you can be sure that performance is being affected:
 

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