db_block_checksum controls whether DBWn and the direct loader will calculate checksums for data blocks and store them in the data blocks' cache header when writing the blocks to disk. db_block_checksum also determines if checksums are calculated for redo log blocks. FULL and the checksum validation fails, Oracle tries to recover the block by reading it from disk or from another instance and then applying redo. db_block_checksum is dependent on db_ultra_safe.