Estimating WAL Log Space Requirements - Teradata Database

Teradata Database Design

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Estimating WAL Log Space Requirements

Teradata Database creates and manages a WAL log that includes the transient journal to store the before-change image of every data row involved in every SQL transaction. This log is used to recover data tables when transactions are aborted.

As transactions are processed, the WAL log grows and shrinks. While transactions are in progress, the WAL log grows according to the total number of data rows being updated or deleted.

The applicable journal rows are purged at intervals (but not before the transaction is completed or, if it was aborted, not before the affected rows are recovered).

Space for the WAL log is acquired from the current PERM space of user DBC. Therefore, it is important that you leave enough PERM space in DBC to accommodate the growth of the largest foreseeable WAL log.

To estimate the maximum size of the WAL log, follow this procedure:

1 Determine the length of the longest row in your production database. Use this figure as your maximum row length.

2 Multiply the maximum row length by the total number of rows in your application programs, batch jobs, and ad-hoc queries that users are likely to update and delete in concurrent transactions.

3 Double the resulting value.

This is the space needed for the WAL log on your system.