5.4. Oracle Settings and Suggestions

Among the thousands of tuning variables in Oracle, the 2 most important are SGA_TARGET and FILESYSTEMIO_OPTIONS. Oracle performs more efficient I/O operations of the files on the GFS volumes are opened with DirectIO (DIO) and AsyncIO (AIO). This is accomplished using the filesystemio_options parameter:

filesystemio_options=setall

filesystemio_options=setall

DirectIO bypasses the page cache for all I/O operations. If DIO is disabled, all datafile I/O will be use the page cache, which effectively double buffers the I/O. Oracle already contains a page cache, called the db block buffer cache. Double buffering increases response time latency for reads, and when the page cache runs the server out of Free memory, system throughput usually drops by 30-50%.

The third most important init.ora parameter must be decided upon first: db_block_size. The default db_block_size for Oracle on Linux is 8K. GFS uses 4K blocks (as does x64 hardware). Although 4K blocks will out-perform 8K blocks in GFS, other factors in the application might mask this effect. Application performance requirements take precedence, and do not change it unless you know what you are doing. It is not recommended that 2K blocks be used on GFS. Most customers leave it 8K.

RAC/GFS was certified using both 4K and 8K blocks, but supports all block size values that the Oracle RDBMS supports.