When you enable block checking, Oracle7 computes a checksum for each block written to disk. Checksums are computed for all data blocks, including temporary blocks.
The DBWR process calculates the checksum for each block and stores it in the block's header. Checksums are also computed by the direct loader.
The next time Oracle7 reads a data block, it uses the checksum to detect corruption in the block. If a corruption is detected, Oracle7 returns message ORA-01578 and writes information about the corruption to a trace file.
Warning: Setting DB_BLOCK_CHECKSUM to TRUE can cause performance overhead. Set this parameter to TRUE only under the advice of Oracle Support personnel to diagnose data corruption problems.