KSDS Random Access: How it Achieves with Free Space

As you’ve already learned, a KSDS consists of a data component and an index component. The primary purpose of the index component is to locate the records in the data component by their key values. To increase the efficiency of this process, the index component consists of an index set and a sequence set.

KSDS Index

As you can see, the sequence set is the lowest level of the index. It’s used to determine which control interval a specific record is stored in. In contrast, the index set consists of records at one or more levels that point to the sequence set records.

The entries in the index set record contain the highest key values stored in each of the sequence set records, and the entries in the sequence set records contain the highest key values stored in each of the control intervals.

Accessing Record

To access a record, VSAM searches from the index set to the sequence set to the control interval that contains the desired record. Then, VSAM reads that control interval into virtual storage and searches the keys of the records until the specific record is found.

The free pointer shown in each sequence set record points to free control intervals in the control area. There is also control information within each control interval (not shown) that identifies the free space in the interval.

By using the free pointers and control information, VSAM is able to manage the free space as it adds records to a KSDS.

VSAM also updates the records in the index and sequence sets as it processes those additions. As a result, the data set can always be used for both sequential and random processing by primary key.


To improve performance, the systems programmer tries to define a production KSDS so all of the index set is brought into internal storage areas called buffers when the data set is processed so that the information can be accessed more quickly. This is one of several ways that the performance of a KSDS can be fine-tuned.

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Author: Srini

Experienced software developer. Skills in Development, Coding, Testing and Debugging. Good Data analytic skills (Data Warehousing and BI). Also skills in Mainframe.