15.00 - 15.10 - Fallback Tables - Teradata Database

Teradata Database Introduction to Teradata

Teradata Database
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User Guide
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English (United States)
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A fallback table is a duplicate copy of a primary table. Each fallback 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 should the system lose an AMP and its associated disk storage in a cluster. In that event, the system would access data in the fallback rows.

The disadvantage of fallback is that this method 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, Teradata Database can repair the primary copy of the data using the fallback copy.

Teradata Database permits the definition of fallback for individual tables. As a general rule, you should run all tables critical to your enterprise in fallback mode. You can run other, non-critical tables in nonfallback mode in order to maximize resource usage.

Even though RAID disk array technology may provide data access even when you have not specified fallback, neither RAID1 nor RAID5 provides 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.