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Oracle AIX Tuning Optimization

Oracle Database Tips by Donald Burleson

Also see these notes on Managing Oracle disk I/O in AIX.

The IBM AIX operating system can be optimized for Oracle databases as noted in this IBM whitepaper titled "Tuning IBM AIX 5L for an Oracle Database".  Also, the TPC benchmarks for Oracle AIX are an excellent was to find optimal Oracle AIX parameter settings.  Also see:

The IBM AIX Oracle whitepaper notes:

Oracle AIX Process Memory tuning

The AIX operating system provides a robust environment for running processes with a wide variety of execution characteristics. By taking advantage of the many variables and features to tune the process environment specifically for the Oracle application, memory usage of each Oracle process is reduced.

The AIXTHREAD_SCOPE environment variable can be used for control if a process runs with process-wide contention scope (the default) or with system-wide contention scope. When using system-wide contention scope, there is a one-to-one mapping between the user thread and a kernel thread. On UNIX? systems, Oracle applications are primarily multiprocess and single-threaded.

One of the mechanisms that enables this multiprocess system to operate effectively is the AIX post/wait mechanism: thread_post(), thread_post_many(), and thread_wait(). It operates most effectively with the Oracle application when using system-wide contention scope (AIXTHREAD_SCOPE=S).

As of AIX V5.2, system-wide contention scope will also significantly reduce the memory required for each Oracle process. For these reasons, always export AIXTHREAD_SCOPE=S before starting all Oracle processes. In Oracle Database 10g a newer, system-wide contention scope is set internally when the instance and user processes are started.

Oracle AIX memory pinning

The AIX operating system allows shared memory to be allocated in pinned memory for improved performance. Pinned memory is always present in real memory; it is not paged in and out. The primary advantage of using pinned memory for the Oracle System Global Area (SGA) is that I/O from pinned memory is optimized for improved path length.

This is possible because the asynchronous I/O routines do not need to pin the SGA memory each time it is used for an I/O operation. Because of this performance improvement, it is best to use pinned memory to allocate the Oracle SGA.
When the Oracle initialization parameter LOCK_SGA is set to TRUE, Oracle will allocate shared memory for the SGA using the AIX pinned memory. (Note: shmget() is called with the SHM_PIN flag set.) In addition to setting the Oracle LOCK_SGA parameter to TRUE, the AIX operating system must be configured to allow pinned memory to be used for shared memory.

AIX Oracle large page support

Give the Oracle user ID the CAP_BYPASS_RAC_VMM and CAP_PROPAGATE capabilities by following these steps:
1. First check the current capabilities: #lsuser ?a capabilities oracle
Note: only the root user can display the capabilities attribute.
2. Add the CAP_BYPASS_RAC_VMM and CAP_PROPAGATE capabilities to the list of capabilities already assigned to this user ID, if any: #chuser capabilities=CAP_BYPASS_RAC_VMM,CAP_PROPAGAE oracle
3. Configure the AIX large page pool by calculating the number of large pages required for the SGA: num_of_large_pages = INT((total_SGA_size-1)/16MB)+1
4. Configure the number and size of large pages: #vmo -p -o lgpg_regions=num_of_large_pages -o lgpg_size=16777216

Oracle AIX I/O Tuning

Asynchronous I/O allows a program to initiate I/O and continue execution of useful work, while other I/O operations are carried out in parallel by the operating system. Because Oracle applications often require multiple server and user processes at the same time, they take advantage of asynchronous I/O to overlap program execution with I/O operations.

Asynchronous I/O is used with Oracle on the AIX operating system to improve system performance.
In AIX V5.2 and newer versions, two asynchronous I/O subsystems are supported, the original asynchronous I/O (called Legacy asynchronous I/O), and the new POSIX asynchronous I/O.

The two types of asynchronous I/O differ in the way the asynchronous I/O subsystem API is defined. Their performance characteristics are the same. All Oracle releases up to and including the current release, Oracle Database 10g, use Legacy asynchronous I/O.

If you like Oracle tuning, you may enjoy the 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 of Oracle tuning scripts.



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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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