Sequence Set,Index Set and Cluster-VSAM

Sequence set
The sequence set is used for sequential access. The sequence set is the lowest level of index CIs and directly points (through an RBA) to the data CI in the CA. There is one index entry per CI data and consequently one index CI for each data CA.

Index set
The index set is used for random access. The index set is the remainder of the index component. If there is more than one sequence set CI, VSAM automatically builds another index CI in other level. Each CI in the index set contains pointers and high key information for CIs in the next lower level of the index.

The index set lists the largest index within each control area and the sequence set lists the largest index within each control interval.

Cluster
A cluster is a set of related components (maximum two). For a KSDS, a cluster is the set of a data component and an index component. The concept of cluster simplifies VSAM processing, providing a way to treat index and data components as a single entity, with its own catalogued name.
RRDS, ESDS, and LDS formats are considered to be clusters without index components. To be consistent, they are given cluster names that are normally used when processing the data set.

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