What is Backup and Recovery?

In general, backup and recovery refers to the various strategies and procedures involved in protecting your database against data loss and reconstructing the database after any kind of data loss.

Physical Backups and Logical Backups

A backup is a copy of data from your database that can be used to reconstruct that data. Backups can be divided into physical backups and logical backups.

Physical backups are backups of the physical files used in storing and recovering your database, such as datafiles, control files, and archived redo logs. Ultimately, every physical backup is a copy of files storing database information to some other location, whether on disk or some offline storage such as tape.

Logical backups contain logical data (for example, tables or stored procedures) exported from a database with an Oracle export utility and stored in a binary file, for later re-importing into a database using the corresponding Oracle import utility.

Physical backups are the foundation of any sound backup and recovery strategy. Logical backups are a useful supplement to physical backups in many circumstances but are not sufficient protection against data loss without physical backups.

Unless otherwise specified, the term "backup" as used in the backup and recovery documentation refers to physical backups, and to back up part or all of your database is to take some kind of physcial backup. The focus in the backup and recovery documentation set will be almost exclusively on physical backups.

Errors and Failures Requiring Recovery from Backup

While there are several types of problem that can halt the normal operation of an Oracle database or affect database I/O operations, only two typically require DBA intervention and media recovery: media failure, and user errors.

Other failures may require DBA intervention to restart the database (after an instance failure) or allocate more disk space (after statement failure due to, for instance, a full datafile) but these situations will not generally cause data loss or require recovery from backup.

 Understanding User Error

User errors occur when, either due to an error in application logic or a manual mis-step, data in your database is changed or deleted incorrectly. Data loss due to user error includes such missteps as dropping important tables or deleting or changing the contents of a table. While user training and careful management of privileges can prevent most user errors, your backup strategy determines how gracefully you recover the lost data when user error does cause data loss.

 Understanding Media Failure

A media failure is the failure of a read or write of a disk file required to run the database, due to a physical problem with the disk such as a head crash. Any database file can be vulnerable to a media failure.

The appropriate recovery technique following a media failure depends on the files affected and the types of backup available.

Oracle Backup and Recovery Solutions: RMAN and User-Managed Backup

For performing backup and recovery based on physical backups, you have two solutions available:

• Recovery Manager, a tool (with command-line client and Enterprise Manager GUI interfaces) that integrates with sessions running on the Oracle server to perform a range of backup and recovery activities, as well as maintaining a repository of historical data about your backups
• The traditional user-managed backup and recovery, where you directly manage the files that make up your database with a mixture of host operating system commands and SQL*Plus backup and recovery-related capabilities

Both methods are supported by Oracle Corporation and are fully documented. Recovery Manager is, however, the preferred solution for database backup and recovery. It can perform the same types of backup and recovery available through user-managed methods more easily, provides a common interface for backup tasks across different host operating systems, and offers a number of backup techniques not available through user-managed methods.

Most of the backup and recovery documentation set will focus on RMAN-based backup and recovery. User-managed backup and recovery techniques are covered in the later chapters of Oracle Database Backup and Recovery Advanced User's Guide.

Whether you use RMAN or user-managed methods, you can supplement your physical backups with logical backups of schema objects made using data export utilities. Data thus saved can later be imported to re-create this data after restore and recovery. However, logical backups are for the most part beyond the scope of the backup and recovery documentation.