In oracle data guard configuration, you need to setup one or more additional databases beside the primary database. These additional databases are called standby database. Up to nice standby database can be created for one primary database.
Using a backup of primary database you can set up standby database and then you can made standby database as part of data guard configuration. Once you configured standby database, data guard automatically maintains standby database by transmitting redo log from the primary database and then applying redo to the standby database.
A standby database can be of three types.
1)Physical Standby Database: A physical standby database is an identical copy of the primary database. The disk structures are also identical with primary database. It is kept synchronized with the primary database by Redo Apply- which means the redo data is received from the primary database and then redo is applied to the physical standby database.
Note that as of Oracle Database 11g release 1 (11.1), a physical standby database can receive and apply redo while it is open for read-only access. You can use physical standby database for query and reporting purpose along with data protection.
2)Logical Standby Database: A logical standby database is the same logical information of the primary database. The physical data structure need not to be same on the standby database. It is kept synchronized with the primary database by SQL Apply- which means the redo data is received from the primary database, transforms redo data into SQL statements and at last executes the SQL statements on the standby database.
You can use logical standby database for query and reporting purpose along with data protection. Also you have to facility to upgrade oracle database software and patch sets along with data protection with help of logical standby database.
3)Snapshot Standby Database: A snapshot standby database is a convertible copy of the physical standby database but the difference from the physical or logical standby database is, the redo data that it received does not apply into it. The redo is applied whenever it is converted back to the physical standby database. You can play with the snapshot standby database and while converting to physical standby database from snapshot standby database these local updates are discarded.
Note that in case of snapshot standby database, the time needed to perform a role transition is directly proportional to the amount of redo data that needs to be applied.
Related Documents
http://arjudba.blogspot.com/2009/04/what-is-oracle-data-guard.html
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