Database Administration - Database Systems - page 344

21.4.1 Oracle Backups: Basic Concept
A backup is a copy of data. This backup may include important parts of the database
such as the control file, datafile(s), or tablespace(s); alternately, it may involve the entire
database. A backup is a safeguard against unexpected data loss and application errors.
If you lose the original data, then you can reconstruct it by using a backup.
Backups are divided into physical backups and logical backups. Physical backups,
which are the primary concern in a backup and recovery strategy, are copies of physical
database files. You can make physical backups with either the Oracle Recovery Manager
(RMAN) utility or operating system utilities. In contrast, logical backups contain logical
data (for example, tables and stored procedures) extracted with the Oracle Export utility
and stored in a binary file. You can use logical backups to supplement physical backups.
21.4.2 Oracle Recovery: Basic Concept
Recovery is the opposite of backup. A database recovery is effected from a database
backup, so if the backup was not done, the recovery is not an option. Like backup,
recovery may involve a component or section of the database (from a control file,
datafile(s) or tablespace(s)), or it may involve an entire database.
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.
21.4.3 Types of Failures
Several circumstances can halt the operation of an Oracle database. The most common
types of failure are described in Figure 21-1 . Oracle provides for complete recovery from
all possible types of hardware failures, including disk crashes. Options are provided so that
a database can be completely recovered or partially recovered to a specific point in time.