User Guide

SAP AG November 2002
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].
Log Backup
In a log backup, the contents of the log area [Page 142] are copied to version files [Page 153]
or backed up using an external backup tool [Page 138].
A version file is generated for each log segment of the log area.
The space originally taken up by the backed up log area becomes free again after the log
backup.
Log Backup Types
The following log backup options are supported for the database system SAP DB:
Automatic log backup [Page
126]
•
• Interactive log backup [Page
139]
Need for log backup
• In the case of a recovery, a data backup [Page 131] alone is not enough to restore the
current state of the database instance [Page 132] up to a certain point in time (for
example, the time just before the disk error occurred).
To restore the current state, both the data backup and the log entries that were written
after the data backup must be imported into the database system.
• If new log entries cannot be written because there is not enough storage space in the
log area, the database stops.
For these reasons, it is necessary to back up the log entries at regular intervals.
See also:
User Manual: SAP DB 142