The log volumes of a database form the log area. The database system permanently stores the redo log entries (after images) in the log area.
The general database parameter MaxLogVolumes specifies the maximum number of log volumes that you can configure for a database (see Database Administration, General Database Parameters).
How Does the System Write Redo Log Entries?
The system distributes the redo log entries among all the log volumes of the log area. The redo log entries are written to the log volumes by the log writer tasks.
LOG FULL Situation
The system overwrites the log area cyclically. However, redo log entries in the log area can only be overwritten once they have been backed up.
If you do not perform any log backups, the database system will at some stage have filled up the log area with redo log entries, and the database comes to a stand-still. Therefore regularly perform log backups or activate automatic log backup.
Database Administration, Backing Up Log Entries
Database Studio, Activating or Deactivating Automatic Log Backup
Database Manager CLI, autolog_on
When the log area is full, you have to perform a log backup. Adding a new log volume does not help: The current write position is usually not the place at which the system adds the new log volume.
The system uses log segments for the automatic log backup. After as many redo log entries as were configured in the log segment size were written to the log area, the system starts a log backup (otherwise after the time specified).
You configure the size of the log segments with the general database parameter AutoLogBackupSize. It is unrelated to the size of the individual log volumes, but may not exceed the total of all log volumes. Make sure that enough space remains in the log area when a log segment is being backed up to receive any new redo log entries that are written during the backup (see Database Administration, General Database Parameters).
You can also activate the overwrite mode for the log area. This means that the database system overwrites the log area without first backing up log entries. Note that this interrupts the backup history, and you can no longer restore the database to its most recent state.
Only switch on overwrite mode if it is not important to you that you can only restore the system to the status of the last available data backup.
For more information, see
Database Studio, Activating or Deactivating the Overwrite Mode of the Log Area
Database Administration in CCMS, Log Area
For the security of your data, we recommend that you mirror the log area. If hardware-based mirroring is not possible, the database system can mirror the log area (software-based mirroring).
The log mode determines whether the database system mirrors the log area (software-based mirroring).
Only mirror the log area using this function if hardware-based mirroring is not possible.
For more information, see