DB_BLOCK_CHECKING

2.76 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.