Database Reference
In-Depth Information
21.4.4 Database Backups
Because one or more files can be physically damaged as the result of a disk failure, media
recovery requires the restoration of the damaged files from the most recent operating
system backup of a database. There are several ways to back up the files of a database.
Whole Database Backups
A whole database backup is an operating system backup of all datafiles, online redo log
files, and the control file of an Oracle database. A whole database backup is performed
when the database is closed and unavailable for use.
Partial Backups
A partial backup is an operating system backup of part of a database. The backup of an
individual tablespace's datafiles and the backup of a control file are examples of partial
backups. Partial backups are useful only when the database's redo log is operated in
ARCHIVELOG mode.
A variety of partial backups can be taken to accommodate any backup strategy.
For example, you can back up datafiles and control files when the database is open or
closed, or when a specific tablespace is online or offline. Because the redo log is operated
in ARCHIVELOG mode, additional backups of the redo log are not necessary. The
archived redo log is a backup of filled online redo log files.
21.4.5 Basic Recovery Steps
Because of the way the Oracle Database Writer (DBWn) writes database buffers to
datafiles, at any given time, a datafile might contain some tentative modifications by
uncommitted transactions and might not contain some modifications by committed
transactions. Therefore, two potential situations can result after a failure:
Data blocks containing committed modifications were not
written to the datafiles, so the changes appear only in the redo
log. Therefore, the redo log contains committed data that must be
applied to the datafiles.
Because the redo log can contain data that was not committed,
uncommitted transaction changes applied by the redo log during
recovery must be erased from the datafiles.
To solve this situation, two separate steps are used by Oracle during recovery from an
instance or media failure: rolling forward and rolling back.
 
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