A fallback table is a duplicate of a primary table. Each row in a fallback table is stored on an AMP different from the one to which the primary row hashes. This storage technique maintains availability if the system loses an AMP and its associated disk storage in a cluster. In that event, the system accesses data in the fallback rows.
The disadvantage of fallback is that it doubles the storage space and the I/O (on INSERT, UPDATE, and DELETE statements) for tables. One advantage is that data is almost never unavailable because of one down AMP. Data is fully available during an AMP or disk outage. Another advantage is that if there is a data read error, Vantage can repair the primary copy of the data using the fallback copy.
Vantage permits the definition of fallback for individual tables. Teradata recommends running tables critical to your enterprise in fallback mode. You can run noncritical tables in nonfallback mode to maximize resource usage.
RAID disk array technology may provide data access even when you have not specified fallback, but it does not provide the same level of protection as fallback.
You specify whether a table is fallback or not using the CREATE TABLE (or ALTER TABLE) statement. The default is not to create tables with fallback.