Oracle Recovery:Basic Concepts
To restore a physical backup of a datafile or control file is to rebuild and make it available on the server to Oracle database. To recover a restored datafile is to update under logging archived redo logs and online, that is, records of changes to the database after the backup was performed. If you use RMAN, you can also recover the restored datafiles with incremental backups(they are backups of a data file that contains only blocks that changed after a previous incremental backup).
Once the necessary files are restored, a media recovery should be initiated by the user. Media recovery can use both archived redo logs and online redo logs to recover data files. If you use SQL * Plus, you can run the RECOVER command to perform recovery. If you use RMAN, then run the RECOVER command RMAN to perform the recovery.
Unlike media recovery, Oracle performs crash recovery and instance recovery automatically after an instance failure. Crash and instance recovery recover a database to a transaction consistent state just before failure, for example. By definition crash recovery, the recovery of a database in a single instance configuration or an Oracle Real Application Clusters configuration in which all instances have crashed. In contrast, the recovery such as recovering a failed instance by a live instance in an Oracle Real Application Cluster configuration.
Crash and instance recovery involve two distinct operations: rolling forward the current, online datafiles by applying both committed and uncommitted transactions contained in online redo records, and then rolling back the changes made in uncommitted transactions to their original state.
To restore a physical backup of a datafile or control file is to reconstruct it and make it available to the Oracle database server. To recover a restored datafile is to update it by applying archived redo logs and online redo logs, that is, records of changes made to the database after the backup was taken. If you use RMAN, then you can also recover restored datafiles with incremental backups, which are backups of a datafile that contain only blocks that changed after a previous incremental backup.
After the necessary files are restored, media recovery must be initiated by the user. Media recovery can use both archived redo logs and online redo logs to recover the datafiles. If you use SQL*Plus, then you can run the RECOVER command to perform recovery. If you use RMAN, then you run the RMAN RECOVER command to perform recovery.
Unlike media recovery, Oracle performs crash recovery and instance recovery automatically after an instance failure. Crash and instance recovery recover a database to its transaction-consistent state just before instance failure. By definition, crash recovery is the recovery of a database in a single-instance configuration or an Oracle Real Application Clusters configuration in which all instances have crashed. In contrast, instance recovery is the recovery of one failed instance by a live instance in an Oracle Real Application Clusters configuration.
Crash and instance recovery involve two distinct operations: rolling forward the current, online datafiles by applying both committed and uncommitted transactions contained in online redo records, and then rolling back changes made in uncommitted transactions to their original state. Because crash and instance recovery are automatic, this manual will not discuss these operations.