User Guide

SAP AG November 2002
Backup Strategy [Page 39]
Saving Log Backups [Page 43]
Log Mode
By specifying the log mode, you can determine how log entries are backed up:
• SINGLE: Log entries are saved to a single log area [Page
142]
• DUAL: Log entries are saved to two log areas in parallel
SINGLE
In the log mode SINGLE, the database instance uses only one log area.
The log area cannot be overwritten until the log entries have been backed up, that is to say, a
log backup [Page 142] has been carried out. This ensures that the database instance [Page
132] can be recovered using a data backup and the related log backups if a disk error leads
to the loss of the log volume content.
DUAL
If you are using RAID-5 or RAID-1 systems, you can set the log mode to SINGLE because
these systems offer sufficient security when in operation. If hardware-based mirroring is not
possible, you should use the log mode DUAL.
In the log mode DUAL, the database instance uses two log areas in parallel.
Data is written to both log areas in parallel. •
• However, during a restart [Page
146] or log backup [Page 142], the log entries are only
read from one log area – the primary log entry.
The two log areas cannot be overwritten until the log entries of the primary log area have
been backed up, that is to say, log backup has been carried out.
If access problems occur in the primary or secondary log area, the log volume in question is
marked as BAD, and the database instance is switched to operating mode [Page
145]
OFFLINE.
The defective log volume of a log area can be recovered in operating mode ADMIN. In this
process, the complete contents of the relevant log volume are copied from the other log area
to the defective log volume.
This ensures that the database instance [Page
132] does not have to be recovered after a
disk error that led to the loss of a log volume, as the content of the log volume was mirrored.
Procedure
Set the database parameter LOG_MODE [Page 84] accordingly. Use the Database Manager
for this.
Database Manager GUI: SAP DB 7.4, Section Changing the Log Mode [See SAP DB Library]
Database Manager CLI: SAP DB 7.4, Section Changing Volume Parameters [See SAP DB
Library]
In rare, special cases, it is possible to disable the writing of log entries as required. In this
case, switch from log mode SINGLE to SINGLE/OFF or from DUAL to DUAL/OFF. Disabling
logging means that the log backup history is interrupted, so if a recovery is necessary, no log
entries are available for this period. You should minimize this security risk, that is to say, you
should keep the period in which there is no active logging as short as possible. Activate
logging by resetting the log mode SINGLE or DUAL. Then carry out a complete data backup
[Page 40].
User Manual: SAP DB 143