User Guide

SAP AG November 2002
138]
CACHE_SIZE [Page 81]
, 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 .
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
, and can be used again by the converter if it grows.
page pool [Page
146]
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
, which is used jointly by the and the data cache. The
size of the I/O buffer cache is determined by the database parameter
. If the converter grows, the number of data cache pages is reduced accordingly.
I/O buffer
cache [Page 138] converter [Page 12 ]9
CACHE_SIZE [Page
81]
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.
Data Volume
A database instance [Page 132] has volumes [Page 153], in which the 15
records the database contents. These volumes are called data volumes. All data volumes
form the
.
data writer [Page ]
data area [Page
131]
Among other things, the data volumes contain the application data [Page
125] and the
metadata of the . database catalog [Page 131]
Managing Application Data
The application data of a table or index can use just one page [Page 146] in the data area as
a minimum. A table can extend across all data volumes of the data area as a maximum. A
table increases or decreases in size automatically without administrative intervention. As a
rule, a database-internal striping algorithm distributes the data belonging to a table evenly
across all the data volumes.
It is therefore not possible or necessary to assign tables to the individual data volumes.
See: Data Management Using B* Trees [Page
101]
Managing Data Volumes
You can configure one or more data volumes when you install a new database instance. The
disk storage space defined by all the data volumes is the total size of the database.
During database operation, you can add new data volumes. The number of data volumes is
limited by the database parameter MAXDATAVOLUMES [Page
85].
See:
Database Manager GUI: SAP DB 7.4, Section
Database Manager CLI: SAP DB 7.4, Section
Managing Database Instances [See SAP DB
Library]
Adding a Volume [See SAP DB Library]
User Manual: SAP DB 130