User Guide

SAP AG November 2002
knldiag.
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 Cache
The catalog cache [Page 129] of a database instance [Page 132] stores the catalog objects
most recently used by a database session [Page 135] and the internal representation
(execution plans) of the most recently executed SQL statements.
Data which is expelled from the catalog cache is moved for the time being to the data cache
[Page 130].
A catalog cache is exists once per database user session.
I/O Buffer Cache
One important cache [Page 129] of a SAP DB database is the I/O buffer cache.
When the database system is started, the I/O buffer cache is created in the main memory in
accordance with the size entered in the database parameter CACHE_SIZE [Page
81] and is
managed via the page pool [Page
146].
A certain number of pages of the I/O buffer cache are made available to the converter [Page
129]. The remaining pages are made available to the data cache [Page 130].
The converter requires more pages during database operation, the distribution of all I/O buffer
cache pages among the converter and data cache is dynamically changes to meet this
requirement.
Converter
The converter is used for the assignment of logical to physical data pages. When data pages
that are not in the data cache [Page 130] are accessed, the system attempts to find their
physical position in the data volumes [Page 130] first in the converter.
User Manual: SAP DB 19