Key Sequenced Data Set
# Key Sequenced Data Set

A key-sequenced data set (KSDS) is a type of data set used by IBM's VSAM computer data storage $system.^{:5}$ Each record in a KSDS data file is embedded with a unique $key.^{:20}$ A KSDS consists of two parts, the data component and a separate index file known as the index component which allows the system to physically locate the record in the data file by its key $value.^{:13}$ Together, the data and index components are called a $cluster.^{:15}$

Records can be accessed randomly or in sequence and can be variable-length.

As a VSAM data set, the KSDS data and index components consist of control $intervals^{:10}$ which are further organized in control $areas.^{:12}$ 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 $intervals.^{:17}$

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 $keys.^{:15}$ An alternate index is itself a $KSDS.^{:16}$

