Informix DBA Survival Guide, Second Edition

Physical and Logical Logs

The physical and logical logs are used by the engine in three major situations:

Transaction Rollback

If a SQL command or a series of SQL commands is executed within a BEGIN WORK/COMMIT WORK structure, the engine will not commit any of the transactions until it receives the OK to "commit work." If something fails, either in a SQL statement or for outside reasons, the engine uses the information in the logical logfiles to roll back the transaction. This brings the database back to a consistent state as though the transaction had never begun.

Restoring from Tape

In the event that the data is completely lost as the result of mechanical or human error and it is necessary to recover the entire system from tape, the tb/ontape ...