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Don Burleson Blog 







db_block_checking tips

Oracle Database Tips by Donald BurlesonJune 15, 2015

The db_block_checking parameter is used to performs block checks for all data blocks making sure that all data in the block is consistent.  The default for db_block_checking is false, because db_block_checking imposes an additional system overhead.

This db_block_checking parameter is used primarily in diagnosing disk I/O issues in system where replicated blocks show corruption.  checks data block integrity before writing the data block to disk.  In this involves checking the internal block headers and footer, plus internal row linkages.

Block checking provides early detection of block corruption, however, it costs between 1-10% in overhead on the database.  The more DML and block writes that occur on a system (INSERT, UPDATE, DELETE), the more costly db_block_checking becomes.

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

Any errors encountered by block checking result in an ORA-600 level message in the alert log.  Before setting the db_block_checking parameter to TRUE, first execute the database verify (dbv) utility against the datafiles to make sure they are free of corruption.   Even when db_block_checking=false, Oracle still provides block checking for the system tablespace.

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

alter system set db_block_checking=true scope=both;

For additional information on handling corruption, see the BC expert notes on corruption and see MOSC Note 1088018.1 - Handling Oracle Database Corruption Issues.

Also see related pages:

- db_block_checksum tips

- db_ultra_safe Tips



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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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