What is a VSAM dataset?

There are three types of datasets access methods used by VSAM. A dataset is a physical record made into a collection which is stored in disks. It is identified by MVS with labels.

• ESDS - Entry Sequence Data Set. This contains all files that are interface specific. It can be accessed through Relative Byte Address.
• KSDS - Key Sequence Data Set. This also contains files that are interface specific. It can be accessed through a secondary index.
• RRDS - Relative Data Set. It contains interface specific files and can be accessed through Relative Record Number.

All the records in VSAM files are either fixed or variable in length. They are all organized in fixed-size blocks which are known as Control Intervals. It is measured in bytes. When the size increases they are called Control Areas. These are measured in disk tracks or cylinders.