Log Mode 

One possible log setting is the log mode. Use this setting to control whether the log area is mirrored or not.

Single Log Area

By default, the database instance uses only one log area.

Even if the database instance only uses one log area, you can still guarantee a high level of security for your database system, since RAID-5 or RAID-1 hardware-based mirroring is usually implemented. RAID systems guarantee a good level of security for production use of the database.

Mirroring the Log Area

If you do not have the option of using hardware-based mirroring, you can use database methods to mirror the log area by setting the log mode accordingly.

When the log area is mirrored, the database instance uses the two log areas as follows:

·        Data is written to both log areas in parallel.

·        However, during a restart or log backup, the log entries are only read from one log area – the primary log entry.

When you use standard log backups, neither of the two log areas can be overwritten until the log entries of the primary log area have been backed up.

If access problems occur in the primary or secondary log area, the log volume in question is marked as BAD, and the database instance is switched to the operational state  OFFLINE.

The defective log volume of a log area can be recovered in the operational state ADMIN. In this process, the complete contents of the relevant log volume are copied from the other log area to the defective log volume.

This ensures that the database instance does not have to be recovered after a disk error that led to the loss of a log volume, as the content of the log volume was mirrored.

Procedure

Set the log mode, and configure the extra log volumes you require. Use the Database Manager for this.
Database Manager GUI: SAP DB 7.4, Section Changing the Log Mode
Database Manager CLI:  SAP DB 7.4, Section Changing Volume Parameters