Basics of Data Guard

Data Guard

An application-transparent high performance low impact online Redo or SQL level background standby database transaction exchange utility capable of reporting, switchover and Failover. Data Guard is a disaster recovery solution for Oracle database. It protects Oracle database against primary site failures, disasters, errors and data corruption. Data Guard can switch any standby database to the production role, minimizing the downtime associated with the outage.

# It helps to take the data and automatically puts it somewhere.

# It makes available for Failover in case of failure.

# It also enable switchover for maintenance

# Helps in reporting

# Backups

Data Guard Configuration

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.

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.

Standby 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.

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. 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

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.

Data Guard Services

There are three types of Data Guard Services as follows:

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.

Role Transitions- 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.

A switchover is a role reversal between the primary database and one of its standby databases. A switchover ensures no data loss.

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.

Data Guard Broker

The Data Guard broker is a distributed management framework that automates the creation, maintenance, and monitoring of Data Guard configurations.

Manager graphical user interface (GUI) or the Data Guard command-line interface (DGMGRL) are used for the configuration of Data Guard.

Data Guard Protection Modes

There are three types of Data Guard Protection Modes-

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.

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.

Data Guard 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.

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.

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.

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.