Oracle Data Guard ensures high availability, data protection, and disaster recovery for enterprise data. Data Guard provides a comprehensive set of services that create, maintain, manage, and monitor one or more standby databases to enable production Oracle databases to survive disasters and data corruptions. Data Guard maintains these standby databases as transactionally consistent copies of the production database. Then, if the production database becomes unavailable because of a planned or an unplanned outage, Data Guard can switch any standby database to the production role, minimizing the downtime associated with the outage. Data Guard can be used with traditional backup, restoration, and cluster techniques to provide a high level of data protection and data availability.
With Data Guard, administrators can optionally improve production database performance by offloading resource-intensive backup and reporting operations to standby systems.
This section contains these topics:
Learn Oracle - Highlights
Learn Oracle - Data Guard Configurations
A Data Guard configuration consists of one production database and one or more standby databases. The databases in a Data Guard configuration are connected by Oracle Net and may be dispersed geographically. There are no restrictions on where the databases are located, provided they can communicate with each other. For example, you can have a standby database on the same system as the production database, along with two standby databases on other systems at remote locations.
You can manage primary and standby databases using the SQL command-line interfaces or the Data Guard broker interfaces, including a command-line interface (DGMGRL) and a graphical user interface that is integrated in Oracle Enterprise Manager.
A Data Guard configuration contains one production database, also referred to as the primary database, that functions in the primary role. This is the database that is accessed by most of your applications.
The primary database can be either a single-instance Oracle database or an Oracle Real Application Clusters database.
A standby database is a transactionally consistent copy of the primary database. Using a backup copy of the primary database, you can create up to nine standby databases and incorporate them in a Data Guard configuration. Once created, Data Guard automatically maintains each standby database by transmitting redo data from the primary database and then applying the redo to the standby database.
Similar to a primary database, a standby database can be either a single-instance Oracle database or an Oracle Real Application Clusters database.
A standby database can be either a physical standby database or a logical standby database:
Physical standby database
Provides a physically identical copy of 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 the same. A physical standby database is kept synchronized with the primary database, though Redo Apply, which recovers the redo data received from the primary database and applies the redo to the physical standby database.
A physical standby database can be used for business purposes other than disaster recovery on a limited basis.
Logical standby database
Contains the same logical information as the production database, although the physical organization and structure of the data can be different. The logical standby database is kept synchronized with the primary database though SQL Apply, which transforms the data in the redo received from the primary database into SQL statements and then executing the SQL statements on the standby database.
A logical standby database can be used for other business purposes in addition to disaster recovery requirements. This allows users to access a logical standby database for queries and reporting purposes at any time. Also, using a logical standby database, you can upgrade Oracle Database software and patch sets with almost no downtime. Thus, a logical standby database can be used concurrently for data protection, reporting, and database upgrades.
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Learn Oracle - Data Guard Services
The following sections explain how Data Guard manages the transmission of redo data, the application of redo data, and changes to the database roles:
Redo Transport Services
Control the automated transfer of redo data from the production database to one or more archival destinations.
Log Apply Services
Apply redo data on the standby database to maintain transactional synchronization with the primary database. Redo data can be applied either from archived redo log files, or, if real-time apply is enabled, directly from the standby redo log files as they are being filled, without requiring the redo data to be archived first at the standby database.
Change the role of a database from a standby database to a primary database, or from a primary database to a standby database using either a switchover or a failover operation.
Redo Transport Services
Redo transport services control the automated transfer of redo data from the production database to one or more archival destinations.
Redo transport services perform the following tasks:
Automatically detect missing or corrupted archived redo log files on a standby system and automatically retrieve replacement archived redo log files from the primary database or another standby database
- Transmit redo data from the primary system to the standby systems in the configuration
- Manage the process of resolving any gaps in the archived redo log files due to a network failure
- Enforce the database protection modes
Log Apply Services
The redo data transmitted from the primary database is written on the standby system into standby redo log files, if configured, and then archived into archived redo log files. Log apply services automatically apply the redo data on the standby database to maintain consistency with the primary database. It also allows read-only access to the data.
The main difference between physical and logical standby databases is the manner in which log apply services apply the archived redo data:
For physical standby databases, Data Guard uses Redo Apply technology, which applies redo data on the standby database using standard recovery techniques of an Oracle database
For logical standby databases, Data Guard uses SQL Apply technology, which first transforms the received redo data into SQL statements and then executes the generated SQL statements on the logical standby database.
An Oracle database operates in one of two roles: primary or standby. Using Data Guard, you can change the role of a database using either a switchover or a failover operation.
A switchover is a role reversal between the primary database and one of its standby databases. A switchover ensures no data loss. This is typically done for planned maintenance of the primary system. During a switchover, the primary database transitions to a standby role, and the standby database transitions to the primary role. The transition occurs without having to re-create either database.
A failover is when the primary database is unavailable. Failover is performed only in the event of a catastrophic failure of the primary database, and the failover results in a transition of a standby database to the primary role. The database administrator can configure Data Guard to ensure no data loss.
The role transitions described in this documentation are invoked manually using SQL statements. You can also use the Oracle Data Guard broker to simplify role transitions and automate failovers using Oracle Enterprise Manager or the DGMGRL command-line interface
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Learn Oracle -
Data Guard Broker
The Data Guard broker is a distributed management framework that automates the creation, maintenance, and monitoring of Data Guard configurations. You can use either the Oracle Enterprise Manager graphical user interface (GUI) or the Data Guard command-line interface (DGMGRL) to:
- Create and enable Data Guard configurations, including setting up redo transport services and log apply services
- Manage an entire Data Guard configuration from any system in the configuration
- Manage and monitor Data Guard configurations that contain Real Application Clusters primary or standby databases
- Simplify switchovers and failovers by allowing you to invoke them using either a single key click in Oracle Enterprise Manager or a single command in the DGMGRL command-line interface.
- Enable fast-start failover to fail over automatically when the primary database becomes unavailable. When fast-start failover is enabled, the Data Guard broker determines if a failover is necessary and initiates the failover to the specified target standby database automatically, with no need for DBA intervention and with no loss of data.
In addition, Oracle Enterprise Manager automates and simplifies:
- Creating a physical or logical standby database from a backup copy of the primary database
- Adding new or existing standby databases to an existing Data Guard configuration
- Monitoring log apply rates, capturing diagnostic information, and detecting problems quickly with centralized monitoring, testing, and performance tools
Using the Data Guard Command-Line Interface
The Data Guard command-line interface (DGMGRL) enables you to control and monitor a Data Guard configuration from the DGMGRL prompt or within scripts. You can perform most of the activities required to manage and monitor the databases in the configuration using DGMGRL. See Oracle Data Guard Broker for complete DGMGRL reference information and examples.
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Learn Oracle - Data Guard Protection Modes
In some situations, a business cannot afford to lose data. In other situations, the availability of the database may be more important than the loss of data. Some applications require maximum database performance and can tolerate some small amount of data loss. The following descriptions summarize the three distinct modes of data protection.
Maximum protection This protection mode ensures that no data loss will occur if the primary database fails. To provide this level of protection, the redo data needed to recover each transaction must be written to both the local online redo log and to the standby redo log on at least one standby database before the transaction commits. To ensure data loss cannot occur, the primary database shuts down if a fault prevents it from writing its redo stream to the standby redo log of at least one transactionally consistent standby database.
Maximum availability This protection mode provides the highest level of data protection that is possible without compromising the availability of the primary database. Like maximum protection mode, a transaction will not commit until the redo needed to recover that transaction is written to the local online redo log and to the standby redo log of at least one transactionally consistent standby database. Unlike maximum protection mode, the primary database does not shut down if a fault prevents it from writing its redo stream to a remote standby redo log. Instead, the primary database operates in maximum performance mode until the fault is corrected, and all gaps in redo log files are resolved. When all gaps are resolved, the primary database automatically resumes operating in maximum availability mode.
This mode ensures that no data loss will occur if the primary database fails, but only if a second fault does not prevent a complete set of redo data from being sent from the primary database to at least one standby database.
Maximum performance This protection mode (the default) provides the highest level of data protection that is possible without affecting the performance of the primary database. This is accomplished by allowing a transaction to commit as soon as the redo data needed to recover that transaction is written to the local online redo log. The primary database's redo data stream is also written to at least one standby database, but that redo stream is written asynchronously with respect to the transactions that create the redo data.
When network links with sufficient bandwidth are used, this mode provides a level of data protection that approaches that of maximum availability mode with minimal impact on primary database performance.
The maximum protection and maximum availability modes require that standby redo log files are configured on at least one standby database in the configuration. All three protection modes require that specific log transport attributes be specified on the LOG_ARCHIVE_DEST_n initialization parameter to send redo data to at least one standby database. See Section 5.6 for complete information about the data protection modes.
If selected blocks within a datafile are corrupt, then you may not have to restore and recover the whole datafile. Instead, you can perform block media recovery. The Recovery Manager BLOCKRECOVER command can restore and recover specified data blocks while the database is open and the corrupted datafile is online.
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Learn Oracle -
Data Guard and Complementary Technologies
Oracle Database provides several unique technologies that complement Data Guard to help keep business critical systems running with greater levels of availability and data protection than when using any one solution by itself. The following list summarizes some Oracle high-availability technologies:
Learn Oracle - Summary of Data Guard Benefits
Data Guard offers these benefits:
- Disaster recovery, data protection, and high availability
Data Guard provides an efficient and comprehensive disaster recovery and high availability solution. Easy-to-manage switchover and failover capabilities allow role reversals between primary and standby databases, minimizing the downtime of the primary database for planned and unplanned outages.
- Complete data protection
Data Guard can ensure no data loss, even in the face of unforeseen disasters. A standby database provides a safeguard against data corruption 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.
- Efficient use of system resources
The standby database tables that are updated with redo data received from the primary database can be used for other tasks such as backups, reporting, summations, and queries, thereby reducing the primary database workload necessary to perform these tasks, saving valuable CPU and I/O cycles. With a logical standby database, users can perform normal data manipulation on tables in schemas that are not updated from the primary database. A logical standby database can remain open while the tables are updated from the primary database, and the tables are simultaneously available for read-only access. Finally, additional indexes and materialized views can be created on the maintained tables for better query performance and to suit specific business requirements.
- Flexibility in data protection to balance availability against performance requirements
Oracle Data Guard offers maximum protection, maximum availability, and maximum performance modes to help enterprises balance data availability against system performance requirements.
- Automatic gap detection and resolution
If connectivity is lost between the primary and one or more standby databases (for example, due to network problems), redo data being generated on the primary database cannot be sent to those standby databases. Once a connection is reestablished, the missing archived redo log files (referred to as a gap) are automatically detected by Data Guard, which then automatically transmits the missing archived redo log files to the standby databases. The standby databases are synchronized with the primary database, without manual intervention by the DBA.
- Centralized and simple management
The Data Guard broker provides a graphical user interface and a command-line interface to automate management and operational tasks across multiple databases in a Data Guard configuration. The broker also monitors all of the systems within a single Data Guard configuration.
- Integration with Oracle Database
Data Guard is a feature of Oracle Database Enterprise Edition and does not require separate installation.
- Automatic role transitions
When fast-start failover is enabled, the Data Guard broker automatically fails over to a synchronized standby site in the event of a disaster at the primary site, requiring no intervention by the DBA. In addition, applications are automatically notified of the role transition.
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