The dual logging option (initialization parameter DUALLOG=Y) protects the database in case of a DASD failure on the log disk. With single logging, any I/O error on the log minidisk causes the database manager to end. With dual logging, database updates are recorded in two log minidisks. This reduces the risk of losing the log, as an unrecoverable error is unlikely to occur on both log minidisks at the same time. The database manager continues running as long as it can read and write from one of the log minidisks.
Chapter 2, Planning for Database Generation describes how to generate a database with either single or dual logging. It also covers various log considerations such as log size and log placement.
To switch between single and dual logging, use the SQLLOG EXEC. For more information, see Reconfiguring and Reformatting the Logs .
Database availability and performance can be improved by making use of the VM Dual Copy function. This function allows two devices attached to the same 3990 Model 3 device to operate in duplex mode. This means that all data written to the primary device is also written to the secondary device. The VM DUPLEX command performs the duplex control operations.
Duplexing the log provides the same benefits as dual logging, but with better performance because it is handled by the operating system. You can also duplex the entire database to provide additional benefits. However, to be effective, all directory disks and dbextents must be duplexed.
| Note: | If the DUPLEX command is used, the PACE parameter is important to ensure continuous database activity during recovery. The PACE parameter indicates the rate at which synchronization of the two volumes is to take place. |
For more information on the Dual Copy function, see the IBM 3990 Storage Control Planning, Installation, and Storage Administration Guide manual. For more information on the DUPLEX command, see the VM/ESA: CP Command and Utility Reference manual.