User Guide

SAP AG November 2002
Undo Log File
During undo log management [Page 47], every modifying transactions [Page 152] creates its
own undo log file, in which the undo log entries [Page
44] are written. Every undo log entries
is assigned an undo log sequence number starting with 0.
Undo log files are internal database storage structures, which are stored in the data area
[Page 131].
Storage of the undo log files in the data area means that the information in the data area can
be used to create a transaction-consistent database state that reflects the state of the
database system at the time of the last savepoint [Page 149].
Delete Undo Log Files
• SAP DB OLTP [Page 149]: At the end of a transaction, the transaction itself deletes the
undo log files.
• liveCache [Page
140]: At the end of the transaction, the undo log files are transferred to
history management [Page
48], because they are needed for unlocked, consistent
reading. The undo log files are deleted at a later point in time by the garbage collector
[Page 49].
In exceptional circumstances, storage of the undo log files in the data area can lead to a full
data area. In this case, increase the size of the data area.
History Management
History management, as a task during online logging [Page 44], is currently only used for the
database instance liveCache [Page 140].
During history management, all information that is needed by the garbage collectors [Page
49] is made available, so that the deleted objects can be completely removed, that is to say,
so that the undo log files [Page 48] can be deleted. In particular, information from the history
files [Page 48] is written to the history list [Page 49].
History File
History files are used during history management [Page 48]. A history file is an internal
database file, which contains the access and statistical information about the undo log files
[Page 48] of transactions [Page 152] that have ended.
When a database instance is created, the initial history file is stored in the data area [Page
131]. The number of history files is defined by the parameter MAXUSERTASKS. Once a
transaction has ended, it writes an entry to its history file. This entry identifies the undo log file
that is assigned to the transaction. With each savepoint [Page 149], the history file is saved to
the data area.
The history list [Page
49] is used to assign the history files to the garbage collectors [Page
49].
User Manual: SAP DB 48