Automatic db_file_multiblock_read_count (MBRC)

Automatic db_file_multiblock_read_count:

The default value corresponds to the maximum I/O size that can be efficiently performed and is platform-dependent

DB_FILE_MULTIBLOCK_READ_COUNT is one of the parameters you can use to minimize I/O during table scans. It specifies the maximum number of blocks read in one I/O operation during a sequential scan. The total number of I/Os needed to perform a full table scan depends on such factors as the size of the table, the multiblock read count, and whether parallel execution is being utilized for the operation.

As of Oracle Database 10g release 2, the default value of this parameter is a value that corresponds to the maximum I/O size that can be performed efficiently. This value is platform-dependent and is 1MB for most platforms.Because the parameter is expressed in blocks, it will be set to a value that is equal to the maximum I/O size that can be performed efficiently divided by the standard block size.
 

According to Oracle, this is the formula for setting db_file_multiblock_read_count:

                                 max I/O chunk size
db_file_multiblock_read_count  = -------------------
                                 db_block_size

MaxMultiblockread count=1MB/8K = 128 (approxmately)

Note that if the number of sessions is extremely large the multiblock read count value is decreased to avoid the buffer cache getting flooded with too many table scan buffers.

Online transaction processing (OLTP) and batch environments typically have values in the range of 4 to 16 for this parameter. DSS and data warehouse environments tend to benefit most from maximizing the value of this parameter. The optimizer is more likely to choose a full table scan over an index if the value of this parameter is high.
The maximum value is the operating system's maximum I/O size expressed as Oracle blocks ((max I/O size)/DB_BLOCK_SIZE). If you set this parameter to a value greater than the maximum, Oracle uses the maximum.

The "real" mbrc is actually somewhere in between since serial multiblock read requests are processed by the buffer cache and split in two or more requests if some blocks are already pinned in the buffer cache, or when the segment size is smaller than the read size.

The mbrc value gathered as part of workload statistics is thus useful for FTS estimation."

One of the new features that was released in 10gR2 is the automatic DB_FILE_MULTIBLOCK_READ_COUNT

The DB_FILE_MULTIBLOCK_READ_COUNT parameter controls the amount of block prefetching done in the buffer cache during scan operations, such as full table scan and index fast full scan. The value of this parameter can have a significant impact on the overall database performance. This feature enables Oracle Database to automatically select the appropriate value for this parameter depending on the operating system optimal I/O size and the size of the buffer cache.
This feature simplifies manageability by automating the tuning of DB_FILE_MULTIBLOCK_READ_COUNT initialization parameter.

This parameter specifies the number of blocks that are read in a single I/O during a full table scan or index fast full scan. The optimizer uses the value of DB_FILE_MULTIBLOCK_READ_COUNT to cost full table scans and index fast full scans. Larger values result in a cheaper cost for full table scans and can result in the optimizer choosing a full table scan over an index scan. If this parameter is not set explicitly (or is set is 0), the optimizer will use a default value of 8 when costing full table scans and index fast full scans.