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Oracle Tips by Burleson

Buffer Performance is critical to Good Oracle Performance

• The parameters used in the parameter file to size the various memory areas.  The test always seems to have a few questions on how to size the different data buffer caches including the Default, Keep and Recycle caches and non-standard blocksize caches (2K, 4K, 8K, etc.)

• Dynamic SGA sizing including a question on the sga_max_size parameter

DBWR_IO_SLAVES vs DB_WRITER_PROCESSES

Questions about multiple DBWR processes have plagued DBAs since Oracle7. You configured multiple DBWR process in Oracle7 by setting the parameter DB_WRITERS. In Oracle7, multiple DBWR processes were actually slave processes that were unable to perform asynchronous I/O calls on their own. The algorithm used by the Oracle7 DBWR caused it to incur waits when the delay of a single write caused additional writes to queue up until the initial write was complete.

Oracle8 and later release’s DBWR architecture corrects this problem.   Oracle’s DBWR now writes continuously without waiting for previous writes to complete.  The new design allows DBWR to act as if it were inherently synchronous, regardless of whether the operating system supports asynchronous I/O or not.  Administrators are able to configure multiple DBWR process by setting the init.ora parameter db_writer_processes. Multiple database writers became available in Oracle 8.0.4 and allow true multiple database writes. There is no master-slave relationship as in Version 7.
 

For more details and scripts, see my new book " Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & scripts.  You can buy it direct from the publisher for 30%-off and get instant access to the code depot.