Control "db file scattered read" IO operations:

Number of blocks read during the "db file scattered read" operation can be controlled through the db_file_multiblock_read_count parameter. The number of blocks within a table's extent needs to be less than or equal to the value specified for db_file_multiblock_read_count for you to see te desired result.

Control ""direct patch write" and "direct path read" IO operations:

The number of blocks for "direct patch write" and "direct path read" operations can be controlled through the hidden parameter _hash_multiblock_io_count.

Example :

SQL> alter session set "_hash_multiblock_io_count"=64;

When set, IO of (_hash_multiblock_io_count) - 1 blocks is performed for direct read / write operations that arise due to hash joins. The value for _hash_multiblock_io_count is supposed to be calculated and set internally by oracle and the parameter value always shows up as 0.

Control all direct IO operations except while sorting and performing hash joins:

Number of blocks read through Direct IO for all operations that has oracle doing direct IO except sort and hash joins can be controlled through the hidden parameter _db_file_direct_io_count. This parameter cannot be changed at the session level and the default value is 1MB (in solaris 10).

Control direct IO operations due to sorting:

Number of blocks read through direct IO due to sort operations can be controlled through _sort_multiblock_read_count paramter. The default value in sun solaris 10 OS is 2 and can be changed at the session level

Important Note : Please keep in mind that simply increasing all / any of the above parameter values in order to increase the number of blocks read per IO operation does not translate to process / query / dml / ddl improvements. On the contrary it could result in performance degradation. Set the applicable values based on your performance improvement analysis and thoroughly test your applications in your test environments.