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. It 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.

Data Guard Fast-Start Failover :Switchover and Failover

Data Guard configuration consists of one database functioning in the production role and one or more databases that function in standby roles. Data Guard maintains these standby databases as synchronized copies of the production database.

A switchover is a planned role reversal between the production database and one of its standby databases to avoid downtime during scheduled maintenance on the production system or to test readiness for future role transitions. A switchover guarantees no data loss. During a switchover, the production database transitions to a standby role, and the standby database transitions to the production role. The transition occurs without having to restart either database.

A failover is performed when the production database (all instances of a RAC production database) fails and one of the standby databases is transitioned to take over the production role, allowing business operations to continue. Once the failover is complete and applications have resumed, the administrative staff can turn its attention to resolving the problems with the failed system.

Fast-Start Failover is an Oracle Data Guard 10g Release 2 feature that quickly and reliably fails over the target standby database to the production database role, without requiring an administrator to perform manual steps to invoke the failover and with no loss of data.

The Observer, which is a Broker client,
- monitors the Data Guard configuration and ensures that it can connect to the production database.
- If Observer and standby lose connectivity to the production database, then Observer will attempt to reconnect for a period of time defined by the administrator.
- If the Observer or Standby database still cannot contact the production database after this period of time has expired, a failover is initiated.
- After failover the Broker automatically reinstates the failed production database as a new target standby once that database is restarted (assuming that the database can be restarted and connectivity to the Observer can be established)

Multiple Data Protection Modes
Maximum performance
•    Maximum protection
•    Maximum availability

Maximum performance was the only data protection mode available prior to Oracle9i. In this mode, either the archiver (ARCn) or the log writer (LGWR) process can be used as the mechanism to initiate the copy of redo from the primary to the standby database. In this mode, the primary database can continue to function even without being able to copy and apply redo information to the standby database.

Maximum protection mode guarantees that transactions committed on the primary will also be committed in the standby database. Data Guard ensures zero data loss when in this configuration. In this mode, the primary database will shutdown if it cannot write to at least one standby database. This mode requires that the LGWR process be used as the mechanism to write the transactions to the standby.

Maximum availability differs from maximum protection mode in one significant way; if the standby database becomes unavailable, the data protection mode is temporarily lowered to maximum performance until the connection to the standby is re-established. This allows you to use the LGWR process to transmit transactions as soon as they occur, but if there is a problem writing to the standby database, it does not stop your primary from continuing to operate.

Good Dataguard Links:

PhysicalStandbyActivatedReadWrite_for_testing.pdf

Data Guard 11g Installation and Configuration On Oracle RAC Systems

Great dataguard presentations from Martin Decker