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Oracle Data Guard Concepts and Administration
Release 2 (9.2)

Part Number A96653-02
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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 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, thus 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.

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 up to nine 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 that 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 another system.

You can manage primary and standby databases using the command-line interface or the Data Guard broker, which includes a graphical user interface called Oracle Data Guard 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 database.

1.1.2 Standby Databases

A standby database is a transactionally consistent copy of the primary database. A standby database is initially created from a backup copy of the primary database. Once created, Data Guard automatically maintains the standby database by transmitting primary database redo data to the standby system and then applying the redo logs 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:

1.1.3 Configuration Example

Figure 1-1 shows a Data Guard configuration that contains a primary database instance that transmits redo data to physical and logical standby databases that are both in remote locations from the primary database instance. In this configuration, a physical standby database is configured for disaster recovery and backup operations, and a logical standby database is configured primarily for reporting, but it can also be used for disaster recovery. You could configure either standby database at the same location as the primary database. However, for disaster recovery, Oracle Corporation recommends that you configure standby databases at remote locations.

Figure 1-1 shows a typical Data Guard configuration in which archived redo logs are being applied to both physical and logical standby databases.

Figure 1-1 Typical Data Guard Configuration

Text description of ls_config.gif follows.

Text description of the illustration ls_config.gif

1.2 Data Guard Services

The following sections explain how Data Guard manages the transmission of redo data, the application of redo logs, and changes to the database roles:

1.2.1 Log Transport Services

Log transport services control the automated transfer of redo data within a Data Guard configuration.

Log transport services perform the following tasks:

1.2.2 Log Apply Services

The redo data transmitted from the primary database is archived on the standby system in the form of archived redo logs. Log apply services automatically apply archived redo logs on the standby database to maintain transactional synchronization with the primary database and to allow transactionally consistent 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 logs:

Log apply services perform the following tasks:

1.2.3 Role Management Services

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. The services that control these aspects are called role management services.

A switchover is a role reversal between the primary database and one of its standby databases. A switchover operation guarantees 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 an irreversible transition of a standby database to the primary role. This is only done in the event of a catastrophic failure of the primary database. The database administrator can configure Data Guard to ensure no data loss.

1.3 Data Guard Broker

The Data Guard broker is a distributed management framework that automates and centralizes the creation, maintenance, and monitoring of Data Guard configurations. The following list describes some of the operations that the broker automates or simplifies:

1.4 Data Guard Protection Modes

In some situations, a business cannot afford to lose data at any cost. 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 a potential loss of data.

Data Guard provides three distinct modes of data protection:

1.5 Summary of Data Guard Benefits

Data Guard offers many overall benefits, as well as benefits provided by each kind of standby database. Note that you need to consider all benefits, including those specific to each type of standby database, when you design your Data Guard configuration.

Data Guard offers these benefits: