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Oracle10g Grid Computing with RAC
Chapter 12 - RAC Design Considerations

Setting dml_locks to Zero

SQL*Loader checks the flag to ensure that there is not a non-parallel direct load running against the same table. The use of direct load forces Oracle to create new extents for each session.

If dml_locks are set to zero on one instance, then it must be set it to zero on all instances. If non-zero values are used with the dml_locks parameter, the values need not be identical on all instances.

Performance for Object Creation in Real Application Clusters

In any Oracle database, DDL statements should be used for maintenance tasks, not during normal system operations. For example, use global temporary tables or PL/SQL tables rather than permanent tables for reports. If this guideline is followed, in most systems, the frequency of DDL statements should be low.

If the application has to create objects frequently, performance degradation to the RAC environment will occur. This is due to the fact that object creation requires inter-instance coordination. A large ratio of dlm_conflicts to dlm_requests on the dc_object_idsrow cache in v$rowcache (the same select as was used for sequences will work here as well), along with excessive wait times for the row cache lock event in v$system_event, is indicative that multiple instances in the cluster are issuing excessive amounts of concurrent DDL statements.

About the only method to improve object creation performance in these situations is to set event 10297 so that it caches OBJECT_ID values. This will improve concurrent object creation by eliminating the recursive DDL and some of the intra-instance pinging. To set event 10297, add the following line to the initialization parameter file:

event="10297 trace name context forever, level 1"

If the additional level argument is set to 1, then the caching behavior is automatically adjustable internally. Otherwise, the level can be set to the desired cache size.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.

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