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For Oracle database, it is mandatory to have 2 or more redo log groups which record the each and every change to the database. The LGWR process writes the changes to the redo log files in a cyclic manner. It starts writing into first redo group and moves to 2nd one and so on. After completion of the last group, the LGWR again starts writing with the first one.
The LGWR pointer change from one log group to another one is called LOG SWITCH. During each log switch the LGWR process overwrite or clean up the new redo log group for the fresh changes.
The LGWR pointer change from one log group to another one is called LOG SWITCH. During each log switch the LGWR process overwrite or clean up the new redo log group for the fresh changes.
How it works?
If your database in archivelog mode, during the log switch the content of the new redo log group will be archived before clean up or overwrite. So depends on the volume of the changes to the database, the number of archivelogs also will be changed. The archiving is done by the ARCh background process. If your database is in noarchivelog mode there will not be any archiving as such. Which means during the log switch the LGWR process simply overwrite the new redolog group.
Advantages & Disadvantages
- Backup: In archivelog mode online / hot backups are possible. Database backups will work when the database is up and running. But in noarchivelog mode only offline backups are possible. Non archivelog mode is not suitable for the 24x7 databases as it needs downtime for the backup. Archivelog mode needed additional storage space for the archive log files and periodic clean up is also needed for the file system or directory where you keep the archivelogs. Each time full backup is not required in case of archivelog mode. Archivelogs can be applied on top of last hot backup and recovery can be achieved. More number of archivelogs after the last hot backup gives more time for the recovery
- Recovery: Point in Time recover is possible with archivelog mode. Database can be recovered upto any past point of time. In non-archivelog mode the recovery is possible upto the last full backup.
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