Media recovery requires that you perform manual tasks to get your database back in one piece. These tasks usually involve a combination of RESTORE and RECOVER commands. You have to issue an RMAN RESTORE command if, for some reason (accidental deleting of files, disk failure, and so on), your data files have experienced media failure.

How the Process Works

When you issue the RESTORE command, RMAN automatically decides how to extract the data files from any of the following available backups:

•     Full database backup

•     Incremental level-0 backup

•     Image copy backup generated by the BACKUP AS COPY command

After the files are restored from a backup, you are required to apply redo to them via the RECOVER command. When you issue the RECOVER command, Oracle examines the SCNs in the affected data files and determines whether any of them need to be recovered. If the SCN in the data file is less than the corresponding SCN in the control file, then media recovery will be required.

Oracle retrieves the data file SCN and then looks for the corresponding SCN in the redo stream to establish where to start the recovery process. If the starting recovery SCN is in the online redo log files, the archivelog files are not required for recovery.

During a recovery, RMAN automatically determines how to apply redo. First, RMAN applies any incremental backups available that are greater than level 0, such as the incremental level 1. Next, any archivelog files on disk are applied. If the archivelog files do not exist on disk, RMAN attempts to retrieve them from a backup set.

To be able to perform a complete recovery, all the following conditions need to be true:

• Your database is in archivelog mode.
• You have a good baseline backup of your database.
• You have any required redo that has been generated since the backup (archivelog files, online redo log files, or incremental backup that RMAN can use for recovery instead of applying redo).

There are a wide variety of restore-and-recovery scenarios. How you restore and recover depends directly on your backup strategy and which files have been damaged. Listed next are the general steps to follow when facing a media failure:

1. Determine which files need to be restored.
2. Depending on the damage, set your database mode to nomount, mount, or open.
3. Use the RESTORE command to retrieve files from RMAN backups. 4. Use the RECOVER command for data files requiring recovery.
4. Open your database.

Your particular restore-and-recovery scenario may not require that all the previous steps be performed. For instance, you may just want to restore your spfile, which does not require a recovery step.

The first step in the restore-and-recovery process is to determine which files have experienced media failure. You can usually determine which files need to be restored from the following sources:

• Error messages displayed on your screen, either from RMAN or from SQL*Plus
• Alert.log file and corresponding trace files

• Data dictionary views

Additionally, to the previously listed methods, you should consider the Data Recovery Advisor for obtaining information about the extent of a failure and corresponding corrective action.