Standby Database Types
A standby database is a transactionally consistent copy of an Oracle production database that is initially created from a backup copy of the primary database. Once the standby database is created and configured, Data Guard automatically maintains the standby database by transmitting primary database redo data to the standby system, where the redo data is applied to the standby database. A standby database can be one of these types: a physical standby database, a logical standby database, or a snapshot standby database. If needed, either a physical or a logical standby database can assume the role of the primary database and take overproduction processing. A Data Guard configuration can include any combination of these types of standby databases.
Physical Standby Databases
A physical standby database is an exact, block-for-block copy of a primary database. A physical standby is maintained as an exact copy through a process called Redo Apply, in which redo data received from a primary database is continuously applied to a physical standby database using the database recovery mechanisms. A physical standby database can be opened for read-only access and used to offload queries from a primary database. If a license for the Oracle Active Data Guard option has been purchased, Redo Apply can be active while the physical standby database is open, thus allowing queries to return results that are identical to what would be returned from the primary database. This capability is known as the real-time query feature.
A physical standby database provides the following benefits:
■Disaster recovery and high availability
A physical standby database is a robust and efficient disaster recovery and high availability solution. Easy-to-manage switch over and fail over capabilities allow easy role reversals between primary and physical standby databases, minimizing the downtime of the primary database for planned and unplanned outages.
A physical standby database can prevent data loss, even in the face of unforeseen disasters. A physical standby database supports all data types, and all DDL and DML operations that the primary database can support. It also provides a safeguard against data corruptions and user errors. Storage level physical corruptions on the primary database will not be propagated to a standby database. Similarly, logical corruptions or user errors that would otherwise cause data loss can be easily resolved.
Oracle Recovery Manager (RMAN) can use a physical standby database to off-load backups from a primary database, saving valuable CPU and I/O cycles. A physical standby database can also be queried while Redo Apply is active, which allows queries to be offloaded from the primary to a physical standby, further reducing the primary workload.
The Redo Apply technology used by a physical standby database is the most efficient mechanism for keeping a standby database updated with changes being made at a primary database because it applies changes using low-level recovery mechanisms which bypass all SQL level code layers.