DB_BLOCK_CHECKSUM介绍

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本文详细介绍了Oracle数据库参数DB_BLOCK_CHECKSUM的作用与设置方法。该参数决定了DBWn和直接加载器是否会在写入磁盘时为每个数据块计算校验和并存储于缓存头部。校验和用于检测由于底层磁盘、存储系统或I/O系统导致的数据损坏。

DB_BLOCK_CHECKSUM

Property	Description
Parameter type	String
Default value	`TYPICAL`
Modifiable	`ALTER SYSTEM`
Range of values	`OFF \| TYPICAL \| FULL (10gR2以后，以前是TRUE OR FALSE)`
Basic	No

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.

If this parameter is set to OFF, DBWn calculates checksums only for the SYSTEM tablespace, but not for user tablespaces.

Checksums allow Oracle to detect corruption caused by underlying disks, storage systems, or I/O systems. If set to FULL, DB_BLOCK_CHECKSUM also catches in-memory corruptions and stops them from making it to the disk. Turning on this feature in TYPICAL mode causes only an additional 1% to 2% overhead. In the FULL mode it causes 4% to 5% overhead. Oracle recommends that you set DB_BLOCK_CHECKSUM to TYPICAL. For backward compatibility we preserve use of TRUE (implying TYPICAL) and FALSE (implying OFF) values.