You can use this log setting to specify whether the log area is mirrored using software-based mirroring. Use this setting to increase the availability of your database instance when no hardware-based mirroring is possible.
Since software-based mirroring has a negative effect on the performance of your database system, we recommend that you use hardware-based mirroring.
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 offer a good level of security for production use of the database.
If you cannot use hardware-based mirroring, then you can use database methods to mirror the log area.
When you use software-based mirroring, the database instance uses the two log areas as follows:
· Data is written to both log areas in parallel.
· During a restart or log backup, the log entries are only read from one log area, the primary log area.
· 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.
· You can restore the defective log volume of a log area in the operational state ADMIN (see Restoring a Mirrored Log Volume). 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 restored after a disk error that led to the loss of a log volume, as the content of the log volume was mirrored.
Use the Database Manager to configure the additional log volumes that are required, and to set the log mode.
Database Manager GUI, Changing Log Settings
Database Manager CLI, Changing Volume Parameters