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Using Concurrent Conventional Path Loads

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

For loading, multiple load sessions executing concurrently on a multiple-CPU system are used. The input data files are split into separate files on logical record boundaries and are then loaded each such input data file with a conventional path load session.This is faster than a single conventional load on a multiple-CPU system, but it is probably not as fast as a direct load. Also, triggers are executed, integrity constraints are applied to the loaded rows, and indexes are maintained using the standard DML execution logic.

Parallel Direct Path Load

SQL*Loader permits multiple, concurrent sessions to perform a direct path load into the same table or into the same partition of a partitioned table. Multiple SQL*Loader sessions improve the performance of a direct path load, given the available resources on the system.

Parallel data loading is enabled by setting both the direct and the parallel parameters to TRUE. However, there are some restrictions to parallel direct load. They are:

* Referential integrity and CHECK constraints must be disabled.* Triggers must be disabled.

* Rows can only be appended. REPLACE, TRUNCATE and INSERT cannot be used.

* Indexes are not maintained during a parallel direct path load. Any table indexes must be created manually or dropped and recreated manually after the load completes.

The following commands invoke three direct path SQL*Loader load sessions on the same table.  These are in separate server sessions:

Session 1:

>sqlldr USERID=nyora/frodo CONTROL=pload1.ctl DIRECT=TRUE PARALLEL=true

Session 2:

>sqlldr USERID=nyora/frodo CONTROL=pload2.ctl DIRECT=TRUE PARALLEL=true

Session 3:

>sqlldr USERID=nyora/frodo CONTROL=pload3.ctl DIRECT=TRUE PARALLEL=true

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Other RAC Parallel Operations

The following are some of the other miscellaneous parallel execution opportunities.


Using multiple DBWRs parallels the writing of buffers. Therefore, multiple DBWR processes should deliver more throughput than one DBWR process with the same number of I/O slaves. Since the mirroring of redo log files on separate disks, does not slow LGWR considerably, LGWR writes to each disk in parallel and waits until each part of the parallel write is complete.

Parallel Recovery

Recovery 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.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

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