What is an UNDO file?
Each transaction which changes data creates a so called UNDO file. This UNDO file contains the information which is necessary for a rollback of a transaction (the before images). As of version 7.4 this UNDO information is kept in the data area and it is even part of a data backup. Therefore a database instance can be restarted even when the log information got corrupted – all open transactions can be rolled back using the information of the UNDO files in the data area. Furthermore all data backups are consistent. Log backups are not needed for a system copy. The UNDO files are deleted at the end of a transaction (OLTP). In a liveCache instance the UNDO information is also used for the consistent read – the garbage collectors delete the UNDO information when it is not needed anymore. Detailed information about the logging concept of MaxDB can be found in the user manual.
