DB_BLOCK_CHECKING

Property	Description
Parameter type	String
Default value	OFF
Modifiable	ALTER SESSION ,  ALTER SYSTEM
Range of values	OFF | LOW | MEDIUM | FULL
Basic	No

DB_BLOCK_CHECKING

 controls whether or not Oracle performs block checking for database blocks. The checking that is performed depends on the value you supply, as follows:

• OFF

   - no block checking is performed for blocks in the user tablespaces. However, semantic blockchecking 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

   or 

  INSERT

   statements, on-disk reads, or inter-instance block transfers in RAC)

• MEDIUM

   - all 

  LOW

   checks are performed, as well as semantic blockchecking for all non-index-organized table blocks

• FULL

   - all 

  LOW

   and 

  MEDIUM

   checks are performed, as well as semantic checks for index blocks(that is, blocks of subordinate objects that can actually be dropped and reconstructed when faced with corruption)

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, depending on workload and the parameter value. 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.

DB_BLOCK_CHECKING=ture，implying 

FULL

，Oracle 数据库将对所有数据块执行块检查。Oracle 数据库通过读取块中的数据来检查块，以确保此数据块的自我一致性。

28. The DB_BLOCK_CHECKING initialization parameter is set to TRUE. What would be the result of this

setting on the data blocks being written to the datafiles, every time the DBWn writes?

A.The Oracle database will check all data blocks by going through the data on each block, making sure

the data is selfconsistent.

B.DBWn and the direct loader will calculate a checksum and store it in the cache header of every data

block when writing it to disk.

C.The Oracle database will check data blocks belonging to the SYSTEM tablespace only, by going

through the data on each block, making sure the data is selfconsistent.

D.The Oracle database will check data blocks belonging to the SYSAUX tablespace only, by going

through the data on each block, making sure the data is selfconsistent.

E.The Oracle database will check data blocks in the SYSTEM and SYSAUX tablespaces only, by going

through the data on each block, making sure the data is selfconsistent.

Answer: A

DB_BLOCK_CHECKING=false; (implying 

OFF

)

 semantic block checkingfor 

SYSTEM

 tablespace blocks is always turned on.只会检查SYSTEM表空间数据块的自我一致性。

27. The DB_BLOCK_CHECKING initialization parameter is set to FALSE. What level of block checking

would be performed?

A.The Oracle database will not perform block checking for any of the data blocks.

B.The Oracle database will perform block checking for the default permanent tablespace only.

C.The Oracle database will perform block checking for the data blocks in all user tablespaces.

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D.The Oracle database will perform block checking for the data blocks in the SYSTEM tablespace only.

E.The Oracle database will perform block checking for the data blocks in the SYSTEM and SYSAUX

tablespaces.

Answer: D