Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 12 BACKUPS
copy of the data in the tables such as is produced by Data Pump Export or a
command.
A logical backup can be much smaller than the corresponding physical backup because
there is typically much unused space within the data blocks and the data files and because
the database contains index data in addition to table data. However, logical backups cannot
be used to restore the database; they can only be used to re-create the data within an otherwise
functional database.
If the database (or part of it) is restored from physical backups, then the redo logs can
be used to recover all modifications to the data made since the physical backup was initiated.
If a logical backup is used to re-create data, any modifications to the data made after
the logical backup was initiated are lost.
Consistent Backups vs. Inconsistent Backups
Many databases are used around-the- clock; if the data in the database is being modified
while the backup is being created, the backup might contain internal consistencies
because each data block in the database is visited just once during a backup operation
and any subsequent changes to the block will not be captured. The only way to guarantee
a consistent backup is to make the database unavailable and prevent changes during the
backup operation. However, inconsistent physical backups are very useful because the
information contained in the redo log files can be used to fix any inconsistencies in such
backups.
Hot vs. Cold Backups
Hot backups (also called online backups) are backups that are created while the database
(or relevant portion thereof) is accessible by users and can be modified while the backup
is underway. Cold backups (also called offline backups) are backups that are created while
the database (or relevant portion thereof) is inaccessible and cannot be modified while
the backup is underway.
Hot backup and online backup are generally considered synonymous with inconsistent
backup, while cold backup and offline backup are considered synonymous with
consistent backup. However, if the database was not shut down gracefully, a cold backup
also may contain inconsistencies. And it is possible to make a consistent logical backup
of the database while the database is online. Finally note that an online backup of a portion
of the database that cannot be modified (for example, a “read- only” table space) is
guaranteed to be consistent.