Overview Of Data Guard Concepts

Data Guard is software that maintains a standby database, or real-time copy of a primary database. Data Guard is an excellent High Availability (HA) solution, and can be used for Disaster Recovery (DR) when the standby site is in a different geographical location than the primary site.

When the sites are identical, and the physical location of the production database is transparent to the user, the production and standby roles can easily switch between sites for many different types of unplanned or planned outages.

Oracle Data Guard manages the two databases by providing remote archiving, managed recovery, switchover and failover features. A secondary site that is identical to the primary site allows predictable performance and response time after failing over or switching over from the primary site. An identical secondary site also allows for identical procedures, processes, and management between sites. The secondary site is leveraged for all unplanned outages not resolved automatically or quickly on the primary site, and for many planned outages when maintenance is required on the primary site.

Data Guard with a physical standby database provides benefits, which fall into two broad classes:
Availability and disaster protection - provides protection from human errors, data failures, and from physical corruptions due to device failure. Provides switchover operations for primary site maintenance, and different database protection modes to minimize or create no data loss environments. A specified delay of redo application at the standby database can be configured to ensure that a logical corruption or error such as dropping a table will be detected before the change is applied to the standby database. Using the standby database, most database failures are resolved faster than by using on-disk backups since the amount of database recovery is dramatically reduced. The standby database can be geographically separate from the primary database, a feature that provides Disaster Recovery against local catastrophic events.
Data Guard, therefore, provides a higher degree of availability than other HA methods that do not employ a second database, such as Real Application Clusters (RAC) or Highly Available Disk Arrays (HADA).

Manageability - provides a framework for remote archiving services and managed standby recovery, contains role management services such as switchover and failover, and allows offloading of backups and read-only activities from the production database. The Data Guard broker provides the Data Guard Manager GUI and command-line interface to automate the management and monitoring of the Data Guard environment.
Operational Requirements
Below are operational requirements for maintaining a standby database. Some of these requirements are more lax then Data Guard best practices would dictate (see Best Practices For Data Guard Configurations below.
· The primary database must run in ARCHIVELOG mode.
· The primary and standby databases must be the same database release.
To use the Data Guard broker, the database server must be licensed for Oracle9i Enterprise Edition or Personal Edition. The operating system on the primary and standby sites must be the same, but the operating system release does not need to be the same. The hardware and operating system architecture on the primary and standby locations must be the same. For example, a Data Guard configuration with a primary database on a 32-bit Linux system must be configured with a standby database on a 32-bit Linux system.
· The primary database can be a single instance database or a multi-instance Real Application Clusters database. The standby databases can be single instance databases or multi-instance Real Application Clusters databases, and these standby databases can be a mix of both physical and logical types.
· If using a physical standby database, log transport services must be configured to specify a dedicated server process rather than a shared server (dispatcher) process in managed recovery mode. Although the read-only mode allows a shared server process, you must have a dedicated server once you open the database again in managed recovery mode.
· The hardware (for example, the number of CPUs, memory size, storage configuration) can be different between the primary and standby systems.
· Each primary database and standby database must have its own control file.
· If you place your primary and standby databases on the same system, you must adjust the initialization parameters correctly.

Much of the material in this section is taken from Oracle9i Data Guard Concepts and Administration.
Oracle9i Data Guard Concepts and Administration, Section 5.1 Introduction to Log Transport Services. This requirement is easy to miss, only found referenced in a Note in this section.