DB_BLOCK_CHECKING

DB_BLOCK_CHECKING specifies whether Oracle AI Database performs block checking for database blocks.

Property	Description
Parameter type	String
Syntax	DB_BLOCK_CHECKING = { FALSE | OFF | LOW | MEDIUM | TRUE | FULL }
Default value	FALSE
Modifiable	ALTER SYSTEM
Modifiable in a PDB	Yes, with the following restriction: If block checking is enabled for a CDB, then you cannot subsequently disable block checking in any of its PDBs. That is, if the value of DB_BLOCK_CHECKING in a CDB is LOW, MEDIUM, TRUE, or FULL, and you then attempt to set the value of DB_BLOCK_CHECKING in one of its PDBs to FALSE or OFF, an error will occur.
Basic	No

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.

Caution: Before enabling block checking with this parameter, Oracle recommends that you detect and repair any logical corruptions in the database. Otherwise, a block that contains logical corruption will be marked as “soft corrupt” after block checking is enabled and the block is modified by a DML statement. This will result in ORA-1578 errors and the block will be unreadable. For more information about detecting and repairing logical corruptions, see Oracle AI Database Backup and Recovery User’s Guide.

See Also: Oracle AI Database Administrator’s Guide for more information about this parameter