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Oracle® Data Guard Concepts and Administration
11g Release 2 (11.2)

Part Number E17022-03
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1 Introduction to Oracle Data Guard

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 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 chapter includes the following topics that describe the highlights of Oracle Data Guard:

1.1 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.

1.1.1 Primary Database

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 (Oracle RAC) database.

1.1.2 Standby Databases

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 thirty 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 RAC database.

The types of standby databases are as follows:

  • 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, through Redo Apply, which recovers the redo data received from the primary database and applies the redo to the physical standby database.

    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. A physical standby database can therefore be used concurrently for data protection and reporting.

  • 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 through SQL Apply, which transforms the data in the redo received from the primary database into SQL statements and then executes 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.

  • Snapshot Standby Database

    A snapshot standby database is a fully updatable standby database.

    Like a physical or logical standby database, a snapshot standby database receives and archives redo data from a primary database. Unlike a physical or logical standby database, a snapshot standby database does not apply the redo data that it receives. The redo data received by a snapshot standby database is not applied until the snapshot standby is converted back into a physical standby database, after first discarding any local updates made to the snapshot standby database.

    A snapshot standby database is best used in scenarios that require a temporary, updatable snapshot of a physical standby database. Note that because redo data received by a snapshot standby database is not applied until it is converted back into a physical standby, the time needed to recover from a primary database failure is directly proportional to the amount of redo data that needs to be applied.

1.1.3 Configuration Example

Figure 1-1 shows a typical Data Guard configuration that contains a primary database that transmits redo data to a standby database. The standby database is remotely located from the primary database for disaster recovery and backup operations. You can configure the standby database at the same location as the primary database. However, for disaster recovery purposes, Oracle recommends you configure standby databases at remote locations.

Figure 1-1 Typical Data Guard Configuration

Description of Figure 1-1 follows
Description of "Figure 1-1 Typical Data Guard Configuration"

1.2 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:

1.2.1 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:

  • 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

  • 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

1.2.2 Apply Services

The redo data transmitted from the primary database is written to the standby redo log on the standby database. 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 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, as shown in Figure 1-2.

Figure 1-2 Automatic Updating of a Physical Standby Database

Description of Figure 1-2 follows
Description of "Figure 1-2 Automatic Updating of a Physical Standby 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, as shown in Figure 1-3.

Figure 1-3 Automatic Updating of a Logical Standby Database

Description of Figure 1-3 follows
Description of "Figure 1-3 Automatic Updating of a Logical Standby Database"

1.2.3 Role Transitions

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.

A failover is when the primary database is unavailable. Failover is performed only in the event of a 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, as described in Section 1.3.

1.3 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:

In addition, Oracle Enterprise Manager automates and simplifies:

See Also:

Oracle Data Guard Broker for more information

1.3.1 Using Oracle Enterprise Manager Grid Control

Oracle Enterprise Manager Grid Control (also referred to as Enterprise Manager in this book) provides a web-based interface for viewing, monitoring, and administering primary and standby databases in a Data Guard configuration. Enterprise Manager's easy-to-use interfaces, combined with the broker's centralized management and monitoring of the Data Guard configuration, enhance the Data Guard solution for high availability, site protection, and data protection of an enterprise.

From the Central Console of Enterprise Manager Grid Control, you can perform all management operations either locally or remotely. You can view home pages for Oracle databases, including primary and standby databases and instances, create or add existing standby databases, start and stop instances, monitor instance performance, view events, schedule jobs, and perform backup and recovery operations.

1.3.2 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.

1.4 Data Guard Protection Modes

In some situations, a business cannot afford to lose data regardless of the circumstances. In other situations, the availability of the database may be more important than any potential data loss in the unlikely event of a multiple failure. Finally, some applications require maximum database performance at all times, and can therefore tolerate a small amount of data loss if any component should fail. The following descriptions summarize the three distinct modes of data protection.

Maximum availability This protection mode provides the highest level of data protection that is possible without compromising the availability of a primary database. Transactions do not commit until all redo data needed to recover those transactions has been written to the online redo log and to the standby redo log on at least one synchronized standby database. If the primary database cannot write its redo stream to at least one synchronized standby database, it operates as if it were in maximum performance mode to preserve primary database availability until it is again able to write its redo stream to a synchronized standby database.

This protection mode ensures zero data loss except in the case of certain double faults, such as failure of a primary database after failure of the standby database.

Maximum performance This is the default protection mode. It provides the highest level of data protection that is possible without affecting the performance of a primary database. This is accomplished by allowing transactions to commit as soon as all redo data generated by those transactions has been written to the online log. Redo data is also written to one or more standby databases, but this is done asynchronously with respect to transaction commitment, so primary database performance is unaffected by delays in writing redo data to the standby database(s).

This protection mode offers slightly less data protection than maximum availability mode and has minimal impact on primary database performance.

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 a transaction must be written to both the online redo log and to the standby redo log on at least one synchronized standby database before the transaction commits. To ensure that data loss cannot occur, the primary database will shut down, rather than continue processing transactions, if it cannot write its redo stream to at least one synchronized standby database.

All three protection modes require that specific redo transport options be used to send redo data to at least one standby database. See Chapter 5, "Data Guard Protection Modes" for more information about setting the protection mode of a primary database.

1.5 Client Failover

A high availability architecture requires a fast failover capability for databases and database clients.

Client failover encompasses failure notification, stale connection cleanup, and transparent reconnection to the new primary database. Oracle Database provides the capability to integrate database failover with failover procedures that automatically redirect clients to a new primary database within seconds of a database failover.

See Also:

1.6 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:

1.7 Summary of Data Guard Benefits

Data Guard offers these benefits: