User Guide

SAP AG November 2002
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
138], which is used jointly by the converter and the data cache. The size of the I/O buffer
cache is determined by the database parameter CACHE_SIZE [Page 81].
If the converter requires more pages than were originally assigned, the number of data cache
pages is reduced accordingly. If the converter requires fewer pages, the free pages are
managed via the page pool [Page
146], and can be used again by the converter if it grows.
Data Cache
The data cache contains the last read- or write-accessed pages of the data volumes [Page
130].
The data cache is shared by all simultaneously active users. It is therefore located in the main
memory.
The data cache is dimensioned dynamically, which means that you cannot directly influence
the size of the data cache. The required data cache pages are taken from the I/O buffer
cache [Page 138], which is used jointly by the converter [Page 129] and the data cache. The
size of the I/O buffer cache is determined by the database parameter CACHE_SIZE [Page
81]. If the converter grows, the number of data cache pages is reduced accordingly.
The hit rate, that is the relation between successful and unsuccessful accesses to the data
cache, is a crucial measure of performance. Successful access means that the required data
was already available in the data cache.
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
1. The user task [Page 16] of the transaction reserves main memory space for a redo log
entry in the log queue.
2. 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).
3. 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
queue to be written to the log area. This behavior is always required for COMMIT [Page
129] and ROLLBACK [Page 147] operations.
If a transaction does wait for the redo log entry to be written, the log writer notifies the
transaction once the relevant page has been written from the log queue to the log area,
User Manual: SAP DB 20