oracle db_ultra_safe Parameter

The new parameter db_ultra_safe sets default values for parameters, such as the db_block_
checking parameter, that control protection levels. To be precise, you can control three corruption  checking parameters—db_block_checking, db_block_checksum, and db_lost_write_protect—by 

specifying values for the db_ultra_safe parameter.

---通过设置db_ultra_safe控制坏块检查参数

The db_ultra_safe parameter can take three values—off, data only, and data and index.

 ---db_ultra_safe可以设置三个值off, data only, and data and index

By default, the db_ultra_safe parameter is set to off, meaning that any values you set for any 
of the three parameters won’t be overridden. If you set the db_ultra_safe parameter value to 
data only, the following will be true:
• db_block_checking will be set to medium.
• db_lost_write_protect will be set to typical.
• db_block_checksum will be set to full.
If you set the db_ultra_safe parameter value to data and index, the following will be true:
• db_block_checking will be set to full.
• db_lost_write_protect will be set to typical.
• db_block_checksum will be set to full.

-----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

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

• MEDIUM - all LOW checks are performed, as well as semantic block checking 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)

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

• MEDIUM - all LOW checks are performed, as well as semantic block checking 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)

  
  

  
  

  --- --DB_LOST_WRITE_PROTECT enables or disables lost write detection. A data block lost write occurs when an I/O subsystem acknowledges the completion of the block write, while in fact the write did not occur in the persistent storage.

  When the parameter is set to TYPICAL on the primary database, the instance logs buffer cache reads for read-write tablespaces in the redo log, which is necessary for detection of lost writes.

  When the parameter is set to FULL on the primary database, the instance logs reads for read-only tablespaces as well as read-write tablespaces.

  When the parameter is set to TYPICAL or FULL on the standby database or on the primary database during media recovery, the instance performs lost write detection.

  When the parameter is set to NONE on either the primary database or the standby database, no lost write detection functionality is enabled.