Disk I/O Integrity Checking
Not all problems with data integrity are the result of keypunch errors or semantic integrity violations. Problems originating in disk drive and disk array firmware can also corrupt user data, typically at the block or sector levels. Block‑ and sector‑level errors are the most common origins of disk I/O corruption encountered in user data.
A major problem with handling this type of data corruption is that it generally is not detected until some time after it has occurred. As a result, queries against the corrupted data return semantically correct, but factually incorrect answer sets, and update or delete operations can either miss relevant rows or can change them in error. Once the system detects the corruption, the affected AMP is typically taken offline, various utilities such as ScanDisk and CheckTable are performed, and the data is either repaired or reloaded. Each of these actions either removes access to, or reduces the availability of, the data warehouse to its users until corrections have been made.