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Lesson 2 Introducing basic database backup concepts
Objective Identify the Available Classes of Database Backup.

Identify the Available Classes of Database Backup using Oracle

There are two types of backup, physical and logical. A physical backup operation results in a byte-for-byte copy of the Oracle database files. Physical backup is also called Operating System backup (O/S) because the DBA copies the files using the O/S, not Oracle.
Logical backup is created using the EXP utility. This utility completely reorganizes the data by extracting it from the physical database file(s) and putting it in a single, "flat" operating system file. The logical backup process is not part of this course.
The images below illustrate the concept for these two types of backup.
Physical and logical backups consisting of INIT, Control, Online Redo Logs and Data Files
Physical and logical backups consisting of INIT, Control, Online Redo Logs and Data Files

Physical backup
A complete physical backup includes all the important parts of a database: control file(s), redo log files, archive log files, and datafiles. If the original data is lost, these file copies can be recovered because the backup contains all the necessary information. In the case ofmedia failure, a physical database backup is the best possible way to successfully recover the data. Without a copy of the database files, not only must the database be re-created empty of user objects, the objects themselves must be re-created. Additionally, all user data must be re-entered. When you make a physical backup of a database, you actually take an operating system[1] backup of datafiles while the database is open or closed. An operating system (O/S) backup is made using an operating system command and can be written to disk or tape in any format that a specific operating system supports.

  1. Media failure: Media failure refers to the error that occurs when you try to write or read a file that is required to operate the database. It is also called disk failure because there is a physical problem reading or writing physical files on the disk.
  2. Mirrored online redo logs: A mirrored online redo log is also called a multiplexed online redo log.
    It contains copies of online redo log files physically located on a separate disk. Any changes made to one member of the group are recorded in other members.

Logical backup

DBAs usually consider logical backup a supplement to their database backup strategy because the EXP utility can be very time-consuming; imagine doing 100 million inserts when the database is large. The EXP utility does not provide the same speed advantages that O/S backups offer at recovery time.
The next lesson examines the available options for database backup.
[1] Operating system backup: An operating system (O/S) backup is made using an operating system command. Operating system backups can be written to disk or tape in any format that a specific operating system supports.