Definition
You can specify how log entries are backed up using the database parameter
LOG_MODE:SINGLE
You only configure one log devspace. This is useful if you only have one disk for the
log area .The log devspace should be backed up periodically.
If a disk fault occurs whereby the contents of the log devspace are destroyed, the
data devspace is no longer consistent with the transaction. You can, however, restore the database instance using a complete data backup and, if necessary, additional log backups. You cannot restore a log area that has not been backed up, nor any transactions that are incomplete.If you are using RAID-5 or RAID-1 systems, you can set the parameter LOG_MODE to SINGLE because these systems offer sufficient security when in operation. Accesses to the log devspace have a large impact on the performance of the database system. For this reason, please ensure when you choose your RAID system that it has a high data throughput.
DUAL
The minimum configuration for log mirroring is two disks, one for the log devspace and one for the mirrored log devspace.
The advantage of this is that if a log devspace fails, it does not interrupt database operation, and once the defective log devspace has been repaired, it can be restored while the database is in operation.
DEMO
If you set the database parameter LOG_MODE to DEMO, the system writes and overwrites its log entries. As soon as the log area is full, the database system overwrites the previous log entries. This means that log entries are lost after a certain amount of time.
For this reason, you can only restore a database instance for which the LOG_MODE database parameter was set to DEMO using a consistent data backup.
Because of the poor safety it offers in the event of a failure, you are advised to use this configuration only for test or demonstration databases.
We recommend you always mirror the log devspaces. You can do this by either using the database software and setting the database parameter LOG_MODE to DUAL, or, if available, using hardware-based mirroring.
If your system allows hardware-based mirroring (RAID-1 systems), we advise you use it.