Log Backup 
In a log backup, the contents of the
log area are copied to version files. A version file is generated for each log segment.
The space originally occupied by the log segments is freed up again after the log backup.
The names of the version files that are generated consist of the version file name entered during definition of the
backup media and a sequence number assigned by the database system when the log segments are saved.
Log Backup Types
The following log backup options are supported for the database system SAP DB:
Automatic Log Backup

You cannot execute log backups in DEMO Log Mode (database parameter LOG_MODE ).
Need for Log Backup
Consistent data backup (for example, a complete data backup with checkpoint in WARM mode is consistent in itself.)
If data is recovered, a consistent data backup is not sufficient to recover the current state of the database instance up to a certain point in time (for example, the time immediately before the disk failure occurred).
In order to recover the current state, the data backup and log entries that were written after the data backup must be restored to the database system.
Inconsistent data backup (for example, a complete data backup without checkpoint in WARM mode is not consistent in itself.)
If data is recovered, you need the relevant log backups for the inconsistent data backup in order to restore a consistent database state.
However, the inconsistent data backup and related log backups are not sufficient to recover the current state of the database instance up to a certain point in time (for example, the time immediately before the disk failure occurred).
In order to recover the current state, the data backup, the related log backups, and the log entries that were written after the data backup must be restored to the database system.
If insufficient storage space in the log area means that new log entries cannot be written, a database standstill results. For this reason, it is necessary to back up log entries regularly.
See also:
Backup Strategy
Saving Log Backups