6.8 Parallel recovery

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In the case of a database crash, instance failure, media failure, etc., Oracle performs a recovery operation against the respective files and areas of Oracle. If these files were located on different disks, then using the parallel recovery option would improve performance of these recovery operations. Parallel recovery divides the recovery operation by allocating different processes to different data blocks during the cache recovery phase of recovery. One example of where this could be used is when applying redo information from the redo logs. During recovery, the redo log is read and blocks that require application of redo log information are parsed out. These blocks are subsequently distributed evenly to all recovery processes to be read into the buffer cache. Similarly, crash, instance, and media recovery of data files on different disk drives are good candidates for parallel recovery.

Based on the type of recovery, there are two methods by which parallel recovery is enabled:

3. Setting the RECOVERY_PARALLELISM parameter to the number of concurrent recovery processes for instance or crash recovery could enable parallel recovery. The value of this parameter should be greater than one and should not exceed the value of the PARALLEL_MAX_SERVERS initialization parameter. This parameter only supports recovery of instance or crash recovery.

4. Setting the PARALLEL clause in the RECOVER DATABASE statement. This clause supports media recovery. Parallel media recovery is controlled by the RECOVER_PARALLEL command that selects a degree of parallelism equal to the number of CPUs available on all participating instances times the value of the PARALLEL_THREADS_PER_CPU initialization parameter. For example, if parallel recovery is performed with PARALLEL 4 and only one data file needs recovery, then four spawned processes read blocks from the data file and apply records.

Media recovery can also be done against the following:

• Database

• Tablespace

• Data file

For example:

RECOVER DATABASE PARALLEL (DEGREE 4 INSTANCES DEFAULT); RECOVER TABLESPACE <tablespace_name> PARALLEL (DEGREE 4 INSTANCES DEFAULT);

More details on database recovery will be covered later in Chapter 12 (Backup and Recovery).

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