ABCs of z/OS System Programming Volume 3 - IBM Redbooks

4.34 VSAM key sequenced cluster (KSDS)
Data component
Figure 4-44 Key sequenced cluster (KSDS)
VSAM KSDS cluster
In a KSDS, logical records are placed in the data set in ascending collating sequence by key.
The key contains a unique value, which determines the record's collating position in the
cluster. The key must be in the same position (off set) in each record.
The key field must be contiguous and each key’s contents must be unique. After it is
specified, the value of the key cannot be altered, but the entire record may be deleted.
When a new record is added to the data set, it is inserted in its logical collating sequence by
key.
A KSDS has a data component and an index component. The index component keeps track
of the used keys and is used by VSAM to retrieve a record from the data component quickly
when a request is made for a record with a certain key.
A KSDS can have fixed or variable length records.
A KSDS can be accessed in sequential mode, direct mode, or skip sequential mode (meaning
that you process sequentially, but directly skip portions of the data set).
166 ABCs of z/OS System Programming Volume 3
VSAM.KSDS
VSAM.KSDS.DATA
VSAM.KSDS.INDEX
Cluster component
Index component