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Unix Backup and Recovery by W. Curtis Preston

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Logical Backups

Physical backups protect you from physical damage, such as a damaged disk drive. A logical backup protects you from logical damage, such as when your DBA accidentally deletes an important table. Logical backups are done by Oracle’s exp utility (short for export), which stores the data in a binary file that is useful only to Oracle. Before deciding whether to implement logical backups for Oracle, though, consider carefully their advantages and disadvantages:

Locates data block corruption

This is probably the biggest advantage to logical backups. They actually examine the logical structure of the database as the export is being performed. Therefore, every data block is examined for consistency. Since this is not done with physical backups, this is the only way to know that there is no corruption of any data blocks anywhere in the database. (Of course, a user attempting to read a bad block would complain as well, but a logical backup reveals this problem before that user needs the block!)

Ability to import a single table

This may or may not be a benefit to everyone, depending on the structure of a given database. If there is a one-to-one relationship of tables to tablespaces, restoring the datafile probably is quicker than restoring from an export. If tables are partitioned across multiple tablespaces, it also is faster to use a physical recovery. However, if there are large tablespaces with multiple tables in them, then it might be much quicker to restore the table than ...

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