A key-sequenced data set (KSDS) is a type of
data set A data set (or dataset) is a collection of data. In the case of tabular data, a data set corresponds to one or more database tables, where every column of a table represents a particular variable, and each row corresponds to a given record of the ...
used by
IBM's
VSAM Virtual Storage Access Method (VSAM) is an IBM DASD file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and no ...
computer data storage
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a compute ...
system.
Each record in a KSDS data file is embedded with a unique key.
A KSDS consists of two parts, the data component and a separate
index file
A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure. Indexes are used to quickly locate data without ...
known as the index component which allows the system to physically locate the record in the data file by its key value.
Together, the data and index components are called a cluster.
Records can be accessed randomly or in sequence and can be variable-length.
As a
VSAM Virtual Storage Access Method (VSAM) is an IBM DASD file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and no ...
data set, the KSDS data and index components consist of control intervals
which are further organized in control areas.
As records are added at random to a KSDS, control intervals fill and need to be split into two new control intervals, each new control interval receiving roughly half of the records. Similarly, as the control intervals in a control area are used up, a control area will be split into two new control areas, each new control area receiving roughly half the control intervals.
While a basic KSDS only has one key (the primary key), alternate indices may be defined to permit the use of additional fields as secondary keys.
An alternate index is itself a KSDS.
See also
*
Entry Sequenced Data Set
*
Linear Data Set
*
Relative Record Data Set
External links
Lascon.co.uk
References
{{Compu-storage-stub
Computer file systems