Tips_30 Detecting and Recovering from Database Corruption
■Parameters Relating to Block Corruption
•DB_BLOCK_CHECKSUM(校验和):defaults to TRUE,will help detect damage introduced by the disk or I/O systems.
Even when this parameter is on false,checksumming is still enabled for the SYSTEM tablespace.
Whenever the DBWn process writes a block to disk it will compute a checksum for the block and include it in the block header.
When a server process reads a block, if the checksum is present it will recalculate it and compare.
This will mean that any damage occurring in the time between the DBWn writing the block and its being read back will be detected.
•DB_BLOCK_CHECKING:defaults to FALSE,will help detect damage introduced by faulty memory.
When set to TRUE, the Oracle processes will check the block for consistency every time the buffer containing the block is accessed.
This will mean that if corruptions are occurring in memory, they will be detected immediately, but the price is high—perhaps as much as 10 percent of processing capacity
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Checksumming and block checking are always enabled for the SYSTEM tablespace, no matter to what these parameters are set.
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Oracle Corporation advises leaving both these parameters on default,unless instructed otherwise by Oracle Support.
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