15.00 - Disk I/O Integrity Checking - Teradata Database

Teradata Database SQL Data Definition Language Detailed Topics

prodname
Teradata Database
vrm_release
15.00
category
Programming Reference
featnum
B035-1184-015K

Disk I/O Integrity Checking

Teradata Database permits you to specify 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.

 

  Rank

Option Name

Checksum Sample Count Percentage

                                  Description

     1

ALL

100

Full end‑to‑end checksums.

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 “Fallback and Hash Indexes” on page 323), 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

 

HIGH

67

Statistically sampled partial end‑to‑end checksums.

Detects lost writes and intermediate levels of bit, byte, and byte string errors.

MEDIUM

33

LOW

2

     3

 

NONE

None

Disables checksum disk I/O integrity checks.

Detects some forms of lost writes using standard file system metadata verification.

DEFAULT

Depends on the setting of the DBS Control Checksum Levels flag.

Percentage specified for this table type in the Checksum Levels fields of the Disk I/O Integrity Fields in the DBS Control utility (see Utilities: Volume 1 (A-K)).

The checksum setting applies to primary and fallback data rows for the table.

This feature detects and logs disk I/O errors: it does not fix them. You cannot rebuild a table using the Table Rebuild utility. Instead, you must drop the table and recreate it.

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.

You can specify system‑wide checksum defaults for various table types using the Checksum Levels fields of the DBS Control 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 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.