User Guide

SAP AG November 2002
Cache
Read and write operations to the volumes [Page 153] of a database instance [Page 132] are
buffered in order to save on disk accesses.
The pertinent main memory structures are called caches. They can be dimensioned
appropriately.
The database system recognizes the following caches, among others:
• Catalog Cache [Page
19]
I/O Buffer Cache [Page
138]
•
Data Cache [Page 130]
Converter [Page
129]
• Log Queue [Page
45]
Catalog
See database catalog [Page 131]
COMMIT
In a COMMIT, all the changes made by a transaction [Page 152] or a
on the are recorded.
subtransaction [See
SAP DB Library] database instance [Page 132]
• Changes closed with a COMMIT can no longer be reversed with a
147 .
ROLLBACK [Page
]
• As a result of a COMMIT, a new transaction is implicitly opened.
In normal database operation, the database system performs the required COMMIT actions
independently. However, COMMIT can also be explicitly requested using appropriate SQL
statements.
In a restart, the system checks which transactions were closed with a COMMIT
statement. These actions are redone.
Converter
The converter is used for the assignment of logical to physical data pages. When data pages
that are not in the are accessed, the system attempts to find their
physical position in the first in the converter.
data cache [Page 130]
data volumes [Page 130]
The converter is used by all users who are active at the same time, and is therefore located in
full in the main memory. Only the converter pages that contain a mapping of permanent data
pages are written to the data volumes with each savepoint [Page
149]. In the case of a restart
[Page 146], the converter can restore the database instance using these pages.
The converter is dimensioned dynamically, which means that you cannot directly influence the
size of the converter. The required converter pages are taken from the I/O buffer cache [Page
User Manual: SAP DB 129