db_block_checksum tips

The db_block_checksum parameter is used to performs block checks for all data blocks making sure that all data in the block is consistent. 

The db_block_checksum, works with db_block_checking and db_ultra_safe.
 
Originally, the db_block_checksum parameter allowed the DBWR (database writer) and sqlldr.exe program to do a checksum when writing data to disk. 

WARNING:  While the Oracle documentation says that db_block_checksum=true only adds a 1 to 10% overhead depending on concurrency of DML, we have seen cases where db_block_checksum=true made the updates run 6x slower. 

In 11g and beyond, the checks are moved from background to foreground processes (IN RAM), creating less overhead then using db_block_checksum against disk.  Hence turning-on block checking will add CPU overhead.

• Setting db_block_checksum=FULL will notify Oracle to verify the checksum as it is being transferred from one memory construct to another (i.e. user session to buffer cache). This improves Oracle's capability of identifying block corruptions and is particularly useful when propagating blocks to standby databases, but adds CPU overhead.
  
• Setting the db_block_checksum=TRUE notifies the database writer process (DBWR) to calculate a value based on the number of bytes in the block and store that value in the cache header of every data block when writing it to disk. This calculated number is called a checksum. Checksums are verified when the block is read and is used by Oracle to detect corruption caused by the various components of disk storage systems.

Note that the db_block_checksum parameter can be changed with an alter system command:

alter system set db_block_checksum=true scope=both;