17.10 - General - Advanced SQL Engine - Teradata Database

Teradata Vantage™ - Database Utilities

Advanced SQL Engine
Teradata Database
Release Number
July 2021
English (United States)
Last Update

You can run SCANDISK while the system is online and Vantage is available for normal operations.

Teradata recommends you run SCANDISK in the following situations:
  • To validate data integrity before or after a system upgrade or expansion.
  • If you suspect data corruption.
  • As a routine data integrity check (perhaps weekly).
A slight performance impact might occur while SCANDISK is running.

You can rearrange the order of the syntax following the SCANDISK command. For example, the command SCANDISK NOCR MI is the same as the command SCANDISK MI NOCR.

If you do not type any options, SCANDISK defaults to DB and all subtables on the vproc. The default scope is to scan both the normal file system and the WAL log, each from the lowest (DB, WDB) level through the highest (MI, WMI). The free CIs are also scanned.

The SCANDISK command can be limited by the SCOPE command to scan, for example, just one table, just the WAL log, or just certain AMPs. For more information, see SCOPE.

By default, SCANDISK uses regular data block preloads instead of cylinder reads. The CR option allows you to run SCANDISK using cylinder reads to preload data into cylinder slots which may improve SCANDISK performance. However, if other work also requires the use of cylinder slots, the competition for slots could slow down SCANDISK and the other work. In addition, the performance gain is dependent on the amount of data loaded, the distribution of the data, and the average block I/O size.

The NOCR option lets you turn off cylinder slot usage by SCANDISK, which could result in slower SCANDISK performance, but which will allow other work requiring cylinder slots to progress unimpeded.

SCANDISK reports only what it finds when scanning is completed.

The output from SCANDISK may include table row identifiers (RowIDs).

A RowID consists of a 16-byte value that includes the following information:
  • The first 8-bytes is the internal partition number of the row.

    (For a nonpartitioned table, the internal partition number is zero, and the internal partition number is not actually stored in the row itself.)

  • The next 8-bytes include a hash bucket value and uniqueness value for the row.
For more information on internal table row formats, see Teradata Vantage™ - Database Design, B035-1094.

SCANDISK start, finish, abort, and system reset times are logged to the Linux /var/log/messages file and also to the DBS.SW_Event_Log table.