User Guide

SAP AG November 2002
• The redo log entries [Page 44] of the transactions are managed: Redo Log Management
[Page 45]
• The undo log entries [Page
44] of the transactions are managed: Undo Log Management
[Page 47]
• For the database instance type liveCache [Page
140], History Management [Page 48] is
carried out for the undo log entries.
Redo Log Management
During online logging [Page 44], the redo log entries [Page 44] are written and managed.
• Active components: transactions [Page
152] in user tasks [Page 16], log writer [Page 46]
• Storage units: log queue [Page
45], log area [Page 142]
Redo log management plays a part in the following actions:
• During execution of a modifying transaction, the redo log entries are included in the log
queue. The log writer writes the log pages from the log queue to the log area.
• During a log backup [Page
142], information is obtained on whether entries from the log
area are to be backed up, and if so, how many.
• For display on the monitor, information is obtained on waiting periods, and the type of
access to the entries in the log queue.
• In the case of a restart or recovery [Page
49], the required redo log entries are read from
the log area.
• The database parameters and other internal information that controls logging are stored
in the log area and updated at regular intervals.
Log Queue
The area of the main memory required for redo log management [Page 45] is called the log
queue. The size of a log queue (in log pages [Page
46]) is determined by the database
parameter LOG_IO_QUEUE [Page
84].
A transaction [Page
152] uses a log queue to obtain a main memory area for a redo log entry
[Page 44]. The transaction writes the redo log entry to the log pages of the log queues.
Writing of the log pages to the log area [Page
142] is carried out by the log writer [Page 46].
Process Flow
4. The user task [Page 16] of the transaction reserves main memory space for a redo log
entry in the log queue.
5. The transaction writes the redo log entry to the reserved area of the log queue.
The time at which the redo log entry is written to the log queue is assigned to the relevant
log page (log queue sequence number).
6. The transaction releases the reserved area of the log queue for processing by the log
writer, and provides information on whether it wants to wait for log page from the log
Use
User Manual: SAP DB 45