Database Reference
In-Depth Information
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 archived redo log 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 archived redo log files on disk
are applied. If the archived redo log 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 (archived redo log
files, online redo log files, or incremental backups 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.
5.
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 doesn't 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 SQL*Plus
•
Alert.log
file and corresponding trace files
Data dictionary views
•
If you're using Oracle 11g or higher, then in addition 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.
Using Data Recovery Advisor
The Data Recovery Advisor tool was introduced in Oracle 11g. In the event of a media failure, this tool will display the
details of the failure, recommend corrective actions, and perform the recommended actions if you specify that it do
so. It's like having another set of eyes to provide feedback when in a restore-and-recovery situation. There are four
modes to Data Recovery Advisor:
•
Listing failures
•
Suggesting corrective action
•
Running commands to repair failures
•
Changing the status of a failure
