Values

• OFF or FALSE

  No block checking is performed for blocks in user tablespaces. However, semantic block checking for SYSTEM tablespace blocks is always turned on.

• LOW

  Basic block header checks are performed after block contents change in memory (for example, after UPDATE, INSERT or DELETE statements, or after inter-instance block transfers in Oracle RAC).

• MEDIUM

  All LOW checks and full semantic checks are performed for all objects except indexes (whose contents can be reconstructed by a drop+rebuild on encountering a corruption).

• FULL or TRUE

  All LOW and MEDIUM checks and full semantic checks are performed for all objects.

Oracle checks a block by going through the data in the block, making sure it is logically self-consistent. Block checking can often prevent memory and data corruption. Block checking typically causes 1% to 10% overhead in most applications, depending on workload and the parameter value. Specific DML overhead may be higher. The more updates or inserts in a workload, the more expensive it is to turn on block checking. You should set DB_BLOCK_CHECKING to FULL if the performance overhead is acceptable.

For backward compatibility, the use of FALSE (implying OFF) and TRUE (implying FULL) is preserved.