Database system with backup and recovery mechanisms

A computer-readable medium storing a program which controls a database system employing a first and second database storage subsystems, the program causing a computer system to function as:

• mirroring means for making the first and second of database storage subsystems operate as a mirrored storage system;
• flag setting means for setting a before-image log saving flag that requests before image log records to be saved in addition to after image log records;
• checkpoint finding means for finding a checkpoint at which it is ensured that all updates of preceding transactions are written in the database storage subsystems;
• detaching means for disabling said mirroring means and detaching the second database storage subsystem from the mirrored storage system after the before image log saving flag is set and the checkpoint is found;
• waiting means for waiting for the completion of such transactions that were in process at the time of the detachment;
• flag clearing means for clearing the before-image log saving flag to stop saving before image log records after said waiting means has finished said waiting operation;
• and
• logging means for creating and storing log records including before-image and after-image logs at the time of the detachment, according to the before-image log saving flag set and cleared by said flag setting means and flag clearing means, respectively.