Disk I/O Integrity Checking
Teradata Database permits you to select various levels of disk I/O integrity checking of your table data using one of the following integrity checks ranked in order of their ability to detect errors.
1 Full end‑to‑end checksums (ALL option)
Detects most bit, byte, and byte string errors.
To enable the file system to detect all hardware read errors for a table and to ensure that the system can use a fallback copy of the data to rebuild a memory‑resident copy of the corrupt data for such errors (see “ALTER TABLE and FALLBACK” on page 42), the CHECKSUM integrity checking level must be set to ALL.
See Database Design for a list of the errors that can be dealt with by the Read From Fallback feature.
2 Statistically sampled partial end‑to‑end checksums (LOW, HIGH, or MEDIUM options)
Detects lost writes and intermediate levels of bit, byte, and byte string errors.
3 Disable checksum disk I/O integrity checks (NONE option)
Detects some forms of lost writes using standard file system metadata verification.
The checksum setting applies to primary data rows, fallback data rows, and all secondary index rows for the table.
You cannot change the checksum level of a dictionary table using ALTER TABLE.
This feature detects and logs disk I/O errors: it does not fix them.
The more words used to generate the checksum value, the better able that checksum is to detect disk I/O errors. Because CPU utilization increases as a function of the number of words used to generate a checksum, several different levels of checking are provided so you can adjust disk I/O integrity checking at the level of individual tables as needed, balancing integrity checking against system performance.
To update the checksum values for all table data blocks and cylinder indexes immediately, specify the IMMEDIATE option. Otherwise, the system only enables checksums gradually as each data block in the table is updated and written to disk. Teradata Database only computes a checksum for a data block when one or more of its rows is modified. When that occurs, the system first generates and then verifies the checksum value for the block.
The following applications are the most common uses for the IMMEDIATE option.
You can specify system‑wide checksum defaults for various table types using the Checksum Levels fields of the DBS Control utility Disk I/O Integrity fields and you can specify the system‑wide checksum level definitions for the LOW, MEDIUM, and HIGH values using the Checksum Level Definitions fields of the DBS Control utility Disk I/O Integrity fields. See Utilities: Volume 1 (A-K) and Database Administration for details.
To enable the file system to detect all single‑bit errors for a table and to ensure that the system can use a fallback copy of the data to recover from such errors, set the CHECKSUM integrity checking level to ALL.