User Guide

SAP AG November 2002
When the automatic log backup [Page 126] is active, the log writer ensures that the redo
log entries from the log area are backed up automatically.
When a certain number of log entries have been written, the administrative information is
copied to the log area, and savepoints are requested, if required. In the case of a restart,
this reduces the restart time.
Log Area
The volume storage area needed for redo log management [Page 45] is called the log area. A
log area can extend across several log volumes [Page
144].
The log area is managed by the log writer [Page 46]. The log writer fills the log area with log
pages [Page 46] from the log queue [Page 45].
For security, the log area should always be mirrored. If possible, you should
mirror the log area on a hardware basis.
If hardware-based mirroring is not possible, you can use the log mode [Page
143]
DUAL.
Ensure that writing of the log entries is always active in a productive database
system.
A full log area leads to a database downtime. Therefore, you must always carry out log
backups [Page 142] in good time. For this purpose, make sure you activate automatic log
backup [Page 126].
In log backups, the log area is not saved as a whole. Instead, it is saved in backup units,
called log segments. The size of a log segment is defined by the parameter
LOG_SEGMENT_SIZE [Page
84].
Undo Log Management
During online logging [Page 44], the undo log entries [Page 44] are managed. The modifying
transactions [Page 152] store the undo log entries in undo log files [Page 48].
Use
Undo log management plays a part in the following actions:
• The database can create a transaction-consistent database state that reflects the state of
the database system at the time of the last savepoint [Page
149]. The information in the
undo log files is sufficient for this.
• In a restart or recovery [Page
49], the required undo log entries are determined.
• For the database instance type liveCache [Page
140], History Management [Page 48] is
carried out for the undo log entries.
User Manual: SAP DB 47