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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 two types: a physical standby database or a logical standby database. If needed, either type of standby database can assume the role of the primary database and take over production processing. A Data Guard configuration can include physical standby databases, logical standby databases, or a combination of both types.
Learn Oracle - Physical Standby Databases
A physical standby database is physically identical to the primary database, with on disk database structures that are identical to the primary database on a block-for-block basis. The database schema, including indexes, are identical.
Data Guard maintains a physical standby database by performing Redo Apply. When it is not performing recovery, a physical standby database can be open in read-only mode, or it can be opened temporarily in read/write mode if Flashback Database is enabled.
The physical standby database is maintained by applying redo data from the archived redo log files or directly from standby redo log files on the standby system using the Oracle recovery mechanism. The recovery operation applies changes in redo blocks to data block using the data-block address. The database cannot be opened while redo is being applied.
A physical standby database can be open in read-only mode so that you can execute queries on the database. While opened in read-only mode, the standby database can continue to receive redo data, but application of the redo data from the log files is deferred until the database resumes Redo Apply.
Although the physical standby database cannot perform both Redo Apply and be opened in read-only mode at the same time, you can switch between them. For example, you can perform Redo Apply, then open it in read-only mode for applications to run reports, and then change it back to perform Redo Apply to apply any outstanding archived redo log files. You can repeat this cycle, alternating between Redo Apply and read-only, as necessary.
The physical standby database is available to perform backups. Furthermore, the physical standby database will continue to receive redo data even if archived redo log files or standby redo log files are not being applied at that moment.
A physical standby database can also be opened for read/write access for purposes such as creating a clone database or for read/write reporting. While opened in read/write mode, the standby database does not receive redo data from the primary database and cannot provide disaster protection.
The physical standby database can be opened temporarily in read/write mode for development, reporting, or testing purposes, and then flashed back to a point in the past to be reverted back to a physical standby database. When the database is flashed back, Data Guard automatically synchronizes the standby database with the primary database, without the need to re-create the physical standby database from a backup copy of the primary database.
Learn Oracle - Benefits of a Physical Standby Database
A physical standby database provides the following benefits:
- Disaster recovery and high availability
A physical standby database enables a robust and efficient disaster recovery and high availability solution. Easy-to-manage switchover and failover capabilities allow easy role reversals between primary and physical standby databases, minimizing the downtime of the primary database for planned and unplanned outages.
- Data protection
Using a physical standby database, Data Guard can ensure no data loss, even in the face of unforeseen disasters. A physical standby database supports all datatypes, 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 do not propagate to the standby database. Similarly, logical corruptions or user errors that cause the primary database to be permanently damaged can be resolved. Finally, the redo data is validated when it is applied to the standby database.
- Reduction in primary database workload
Oracle Recovery Manager (RMAN) can use physical standby databases to off-load backups from the primary database saving valuable CPU and I/O cycles. The physical standby database can also be opened in read-only mode for reporting and queries.
The Redo Apply technology used by the physical standby database applies changes using low-level recovery mechanisms, which bypass all SQL level code layers; therefore, it is the most efficient mechanism for applying high volumes of redo data.
Learn Oracle - Logical Standby Databases
A logical standby database is initially created as an identical copy of the primary database, but it later can be altered to have a different structure. The logical standby database is updated by executing SQL statements. This allows users to access the standby database for queries and reporting at any time. Thus, the logical standby database can be used concurrently for data protection and reporting operations.
Data Guard automatically applies information from the archived redo log file or standby redo log file to the logical standby database by transforming the data in the log files into SQL statements and then executing the SQL statements on the logical standby database. Because the logical standby database is updated using SQL statements, it must remain open. Although the logical standby database is opened in read/write mode, its target tables for the regenerated SQL are available only for read-only operations. While those tables are being updated, they can be used simultaneously for other tasks such as reporting, summations, and queries. Moreover, these tasks can be optimized by creating additional indexes and materialized views on the maintained tables.
A logical standby database has some restrictions on datatypes, types of tables, and types of DDL and DML operations.
Learn Oracle - Benefits of a Logical Standby Database
A logical standby database provides similar disaster recovery, high availability, and data protection benefits as a physical standby database. It also provides the following specialized benefits:
- Efficient use of standby hardware resources
A logical standby database can be used for other business purposes in addition to disaster recovery requirements. It can host additional database schemas beyond the ones that are protected in a Data Guard configuration, and users can perform normal DDL or DML operations on those schemas any time. Because the logical standby tables that are protected by Data Guard can be stored in a different physical layout than on the primary database, additional indexes and materialized views can be created to improve query performance and suit specific business requirements.
- Reduction in primary database workload
A logical standby database can remain open at the same time its tables are updated from the primary database, and those tables are simultaneously available for read access. This makes a logical standby database an excellent choice to do queries, summations, and reporting activities, thereby off-loading the primary database from those tasks and saving valuable CPU and I/O cycles.
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We learned Standby database types today. Next we will move to User Interfaces for Administering Data Guard Configurations.
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