DB_BLOCK_CHECKSUM

Property	Description
Parameter type	String
Syntax	DB_BLOCK_CHECKSUM = { OFF | FALSE | TYPICAL | TRUE | FULL }
Default value	TYPICAL
Modifiable	ALTER SESSION, ALTER SYSTEM
Basic	No

DB_BLOCK_CHECKSUM determines whether DBWn and the direct loader will calculate a checksum (a number calculated from all the bytes stored in the block) and store it in the cache header of every data block when writing it to disk. Checksums are verified when a block is read - only if this parameter is TYPICAL or FULL and the last write of the block stored a checksum. In FULL mode, Oracle also verifies the checksum before a change application from update/delete statements and recomputes it after the change is applied. In addition, Oracle gives every log block a checksum before writing it to the current log.

Starting with Oracle Database 11g, most of the log block checksum is done by the generating foreground processes, while the LGWR performs the rest of the work, for better CPU and cache efficiency. Prior to Oracle Database 11g, the LGWR solely performed the log block checksum. When this parameter is set to FULL, the LGWR verifies the checksum of each log block generated by the foreground processes before writing it to disk.

If this parameter is set to OFF, DBWn calculates checksums only for the SYSTEM tablespace, but not for user tablespaces. In addition, no log checksum is performed when this parameter is set to OFF.

Checksums allow Oracle to detect corruption caused by underlying disks, storage systems, or I/O systems. If set to FULL, DB_BLOCK_CHECKSUM also catches in-memory corruptions and stops them from making it to the disk. Turning on this feature in TYPICAL mode causes only an additional 1% to 2% overhead. In the FULL mode it causes 4% to 5% overhead. Oracle recommends that you set DB_BLOCK_CHECKSUM to TYPICAL.

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

Note:

1. Related parameter: DB_BLOCK_CHECKING

2. This parameter was introduced in Oracle 7.2.

3. See Metalink note 32969.1 for details of this and other block checking features in various Oracle releases.

4. Query for the current value of the parameter

select name, value, isdefault, isses_modifiable, issys_modifiable,

isinstance_modifiable, isdeprecated, description

from v$parameter where upper(name) = ‘DB_BLOCK_CHECKSUM’;

NAME	VALUE	IS DEFAULT	ISSES_ MODIFIABLE	ISSYS_ MODIFIABLE	ISINSTANCE_ MODIFIABLE	IS DEPRECATED	DESCRIPTION
db_block_checksum	TRUE	TRUE	FALSE	IMMEDIATE	TRUE	FALSE	store checksum in db blocks and check during reads

Oracle initializatoin parameters