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 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.