User Guide

SAP AG November 2002
Database Instance
The SAP DB database can be installed and run on a computer in one mode (database
instance) or several modes (database instances) (Database Instance Type [Page
132], see
also: SAP DB Versions and Database Instance Types [Page 24]).
Every database instance consists of threads [Page
151], main memory structures (caches
[Page 129]) and volumes [Page 153].
Database
instance
Threads
Caches
Volumes
The following volumes are available for each database instance:
• One or more data volumes [Page
130]
• One or more log volumes [Page
144]
Each database instance differentiates between the following areas for the logical storage of
data:
• Database catalog [Page
131]
• Application data [Page
125]
Thread
A whole series of operating system threads (often referred to as kernels) belong to a
database instance [Page
132].
We differentiate between UKTs (user kernel threads) and special threads.
The required number of UKTs and of special threads depends on the hardware configuration,
the number of volumes [Page
153] used, and the database parameters that were set.
• User Kernel Thread (UKT) [Page
14]
• Special Thread [Page
17]
• Operating-System-Dependent Special Thread [Page
18]
User Kernel Thread (UKT)
A database instance [Page 132] contains a series of threads [Page 151]. A user kernel thread
(UKT) forms a subset of all tasks [Page
151] (for internal tasking).
The following types of tasks exist:
• Data Writer [Page
15]
User Manual: SAP DB 14