Oracle Data Guard (known as Oracle Standby Database prior to Oracle9i), forms an extension to the Oracle RDBMS and provides organizations with high availability, data protection, and disaster recovery for enterprise databases.
One of those new features in Oracle9i Release 2 is the ability for the DBA to place the database into one of the following protection modes:
- Maximum Protection
- Maximum Availability
- Maximum Performance
A Data Guard configuration will always run in one of the three protection modes listed above. Each of the three modes provide a high degree of data protection; however they differ with regards to data availability and performance of the primary database.
Log Transport Services enables and controls the automated transfer of redo data within a Data Guard configuration from the primary site to each of its standby sites.
Log transport services also controls the level of data protection for your database. The DBA will configure log transport services to balance data protection and availability against database performance. Log transport services will also coordinate with Log Apply Servicesand Role Transition Services for switchover and failover operations.
Maximum Protection mode offers the ultimate in data protection. It guarantees no data loss will occur in the event the primary database fails. In order 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 a standby redo log on at least one standby database before the transaction can be committed. In order to guarantee no loss of data can occur, the primary database will shut down if a fault prevents it from writing its redo data to at least one remote standby redo log.
In a multiple-instance RAC database environment, Data Guard will shut down the primary database if it is unable to write the redo data to at least one properly configured database instance (see minimum requirements below).
In order to participate in Maximum Protection mode:
- At least one standby instance has to be configured with standby redo logs.
- When configuring the standby destination service in the LOG_ARCHIVE_DEST_n initialization parameter on the primary database, you must use the LGWR, SYNC, and AFFIRM attributes.
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Maximum Availability Mode
Maximum Availability mode provides the highest level of data protection that is possible without affecting the availability of the primary database. This protection mode is very similar to Maximum Protection where a transaction will not commit until the redo data needed to recover that transaction is written to both the local (online) redo log and to at least one remote standby redo log. Unlike Maximum Protection mode; however, the primary database will not shut down if a fault prevents it from writing its redo data to a remote standby redo log. Instead, the primary database will operate in Maximum Performance mode until the fault is corrected and all log gaps have been resolved. After all log gaps have been resolved, the primary database automatically resumes operating in Maximum Availability mode.
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Just like Maximum Protection mode, Maximum Availability requires:
- At least one standby instance has to be configured with standby redo logs.
- When configuring the standby destination service in the LOG_ARCHIVE_DEST_n initialization parameter on the primary database, you must use the LGWR, SYNC, and AFFIRM attributes.
Maximum Performance Mode
Maximum Performance mode is the default protection mode and provides the highest level of data protection that is possible without affecting the performance or availability of the primary database. With this protection mode, a transaction is committed as soon as the redo data needed to recover the transaction is written to the local (online) redo log.When configuring the standby destination service in the LOG_ARCHIVE_DEST_n initialization parameter on the primary database, log transport services can be set to use either LGWR / ASYNC or ARCH. In order to reduce the amount of data loss on the standby destination if the primary database were to fail, set the LGWR and ASYNC attribute. Using this configuration, the primary database writes its redo stream to the standby redo logs on the standby database asynchronously with respect to the commitment of the transactions that create the redo data. When the nodes are connected with sufficient bandwidth, this mode provides a level of data protection that approaches that of Maximum Availability mode with minimal impact on primary database performance.
Note that the use of standby redo logs while operating in Maximum Performance mode is only necessary when configuring log transport services to use LGWR. When log transport services is configured to use ARCH, standby redo logs are not required.
SQL> alter database set standby database to maximize protection;
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SQL> SELECT PROTECTION_MODE, PROTECTION_LEVEL, DATABASE_ROLE FROM V$DATABASE;
PROTECTION_MODE PROTECTION_LEVEL DATABASE_ROLE
------------------- ------------------- --------------
MAXIMUM PROTECTION MAXIMUM PROTECTION PRIMARY
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SUMMARY :
1) Maximum protection ( zero data loss)
· Redo synchronously transported to standby database
· Redo must be applied to at least one standby before transactions on primary can be committed
· Processing on primary is suspended if no standby is available
2) Maximum availability (minimal data loss)
· Similar to maximum protection mode
· If no standby database is available processing continues on primary
3) Maximum performance (default)
· Redo asynchronously shipped to standby database
· If no standby database is available processing continues on primary
Read More: TRANSPORT: Data Guard Protection Modes (Doc ID 239100.1)
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