DB_BLOCK_CHECKSUM介绍
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DB_BLOCK_CHECKSUM
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 DBW
n
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
, DBW
n
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.
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