Data Guard - Session 1
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. (If a standby database is located on the same system as the primary database, the archival directories for the standby database must use a different directory structure than 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.
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.
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
■ 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.
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:
■ Create and enable Data Guard configurations, including setting up redo transport services and log apply services
■ Manage an entire Data Guard configuration from any system in the configuration
■ Manage and monitor Data Guard configurations that contain Real Application Clusters primary or standby databases
■ Simplify switchovers and failovers by allowing you to invoke them using either a single key click in Oracle Enterprise Manager or a single command in the DGMGRL command-line interface.
■ Enable fast-start failover to fail over automatically when the primary database becomes unavailable. When fast-start failover is enabled, the Data Guard broker determines if a failover is necessary and initiates the failover to the specified target standby database automatically, with no need for DBA intervention and with no loss of data.
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.
This mode ensures that no data loss will occur if the primary database fails, but only if a second fault does not prevent a complete set of redo data from being sent from the primary database to at least one standby database.
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.
Hardware and Operating System Requirements
All members of a Data Guard configuration must run an Oracle image that is built for the same platform.
The hardware (for example, the number of CPUs, memory size, storage
configuration) can be different between the primary and standby systems.
Oracle Software Requirements
Oracle Data Guard is available only as a feature of Oracle Database Enterprise
Edition. It is not available with Oracle Database Standard Edition. This means the same release of Oracle Database Enterprise Edition must be installed on the primary database and all standby databases in a Data Guard configuration.
The COMPATIBLE initialization parameter must be set to the same value on all databases in a Data Guard configuration.
The primary database must run in ARCHIVELOG mode.
Oracle recommends that when you set up Oracle Automatic Storage Management (ASM) and Oracle Managed Files (OMF) in a Data Guard configuration, set it up symmetrically on the primary and standby database. That is, if any database in the Data Guard configuration uses ASM, OMF, or both, then every database in the configuration should use ASM, OMF, or both, respectively