DB_BLOCK_CHECKSUM determines whether
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
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.
Starting with Oracle Database 11g, most of the log block checksum is done by the generating foreground processes, while the LGWR performs the rest of the work, for better CPU and cache efficiency. Prior to Oracle Database 11g, the LGWR solely performed the log block checksum. When this parameter is set to
FULL, the LGWR verifies the checksum of each log block generated by the foreground processes before writing it to disk.
If this parameter is set to
n calculates checksums only for the
SYSTEM tablespace, but not for user tablespaces. In addition, no log checksum is performed when this parameter is set to
Checksums allow Oracle to detect corruption caused by underlying disks, storage systems, or I/O systems. If set to
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
For backward compatibility the use of
OFF) values is preserved.