Entry sequential data set (ESDS)
• Records in an ESDS are stored in the order in which they are written and are retrieved by addressed access
• Records are loaded irrespective of their contents and their byte address cannot be changed
• ESDS is also referred to as a sequential VSAM data set. This is because records in an ESDS are normally processed sequentially
• ESDS is best suited for applications where most processing is done sequentially
Key Sequential data set (KSDS)
• Records in an KSDS are stored in key sequence and are controlled by an index, determine the order in which records are stored
• In a KSDS, records can be processed both sequentially and randomly using their key field values
• Advantages – sequential processing in useful retrieval of records in sorted form
• Random or direct processing of records useful in online applications
Relative record data set (RRDS)
• Records in an RRDS are loaded into fixed-length or variable length slots
• These records are represented by relative record numbers (RRNs) of their slots
• A processing program uses RRN to provide random access
Linear data set (LDS)
• Data set containing contiguous string of data bytes with no intervening control information E.g., Source control data set (SCDS), Active Control data set (ACDS), Communication data set (COMMDS)
• An LDS is divided into blocks. These blocks can be sequentially retrieved by a processing program in physical order
• Processing program can group several logical records together in to a single block
• LDS can be kept permanently in store for enhanced performance
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