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Don Burleson Blog 







Oracle parallel recovery tips

Oracle Database Tips by Donald BurlesonJuly 18,  2015

Question:  I want to maximize the speed of my RMAN recovery, and I want to employ parallelism for the recovery process.  How does parallel recovery relate to the number of devices?

Answer:  Back before RMAN, the degree of parallelism depended upon the number of devices that could re-apply the data files in parallel.  For example a parallel recovery from ten backups could be run in parallel if ten separate devices should simultaneously re-apply the data files (assuming on disk contention). 

Recoveries performed in parallel can speed up the crash, instance, and media recovery considerably. During a parallel recovery process, the SMON background process reads the redo-log files sequentially, and the redo-blocks are then distributed evenly to all recovery processes to be read into the buffer cache. The parallel slave processes apply the changes to data files. If the data files involved in the recovery are many, the parallel process really helps.

To specify the number of concurrent recovery processes for instance or crash recover use the recovery_parallelism initialization parameter to specifies the number of concurrent recovery processes. This parameter has no affect on media recovery. To parallelize media recovery, use the parallel clause in the RECOVER DATABASE statement.

The SQL*Plus recovery_parallelism command specifies parallel media recovery. The default is NOPARALLEL.



For media recovery, Oracle uses a division of labor approach to allocate different processes to different data blocks while rolling forward, thereby making the procedure more efficient. For example, if parallel recovery is performed with parallel four, and only one data file is recovered, four spawned processes read blocks from the data file and apply records instead of only one process.

Recovery with Oracle Database 10g is automatically parallelized for these three stages of media recovery:

* Restoration of data files.

* Application of incremental backups.

* Application of redo logs.

The number of channels configured for a database in RMAN determines the degree of parallelism for data file restoration. In the previous example configuration, two streams could have been involved in restoring data files since two channels were configured. The degree of parallelism for restoration of incremental backups is also dependent on the number of configured channels.

Redo logs are applied by RMAN using the degree of parallelism specified in the initialization parameter recovery_parallelism.

In Oracle Database 10g, there is no server manager program, so all DBA functions are done through SQL*PLUS. Using manual recovery methods such as SQL*PLUS, values can be specified for recovery_parallelism, since it is a dynamic parameter. However, it cannot exceed the setting for parallel_max_servers. Using the DEGREE option for the RECOVER command, the degree of parallelism can also be controlled for other recovery operations.

Also see these related notes on parallel recovery:




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