Introduction (for Beginners)
- There are several access methods that are developed to access the data from the files. VSAM is the most used and one of the several access methods using by OS of mainframe.
- VSAM is the short form of Virtual Storage Access Method. VSAM was introduced in the early 1970s by IBM with three dataset organizations - sequential, indexed, and direct access.
What is VSAM?
- Virtual Storage Access Method (VSAM) refers to both type of the dataset and the access methods that are used to manage various dataset types.
- VSAM is a type of record-oriented system and an IBM invented access method that allows us to access and organize records in the dataset.
- The difference between other access methods and VSAM is, VSAM maintains disk records in a unique format using complex functions that is not understandable by any other access methods.
- VSAM works with the data that stores only on direct access storage devices (DASDs). i.e., disks.
VSAM Dataset Types
Based on the VSAM storage format, accessing, and organizing the records, the datasets are classified into four different types. Those are -
- Entry-Sequenced Data Sets (ESDS)
- Key-Sequenced Data Sets (KSDS)
- Relative Record Data Sets (RRDS)
- Linear Data Sets (LDS)
The primary difference between these dataset types is - the way the records are stored and accessed.
VSAM Access and retrieval options
VSAM provides three types of record accessing in VSAM datasets -
- Dynamic or Skip sequential
Access Methods and File Organization
From the basic dataset definitions, dataset types are classified based on the several ways of organizing the data on IBM disks. IBM software supports the below dataset types as of today -
- Physical Sequential (PS)
- Partitioned Organization (PO)
- Indexed Sequential (IS)
- Direct Access (DA)
- Virtual Storage Access Method (VSAM)
Each dataset type except VSAM has its own records organizing method depending on its purpose. However, VSAM provides a functional equivalent for most of the existing dataset types as follows -
- ESDS is the functional equivalent of Physical Sequential organization (PS).
- KSDS is the functional equivalent of Indexed Sequential organization (IS).
- RRDS is the functional equivalent of Direct Access organization (DA).
How VSAM is different from other datasets?
VSAM datasets differ from other dataset types for the below two reasons -
- VSAM datasets are device-independent.
- VSAM datasets can be both sequentially and directly accessed.
- High-speed data retrieval when compared to other access methods because of its organized index.
- Supports all types of data retrieval (sequential, random, dynamic, linear).
- Supports fixed and variable-length records processing.
- Supports in batch and online systems.
- Records can be deleted physically and can reuse the deleted records space to store new records without reorganization. It helps for faster retrieving.
- Supports alternate index.
- Improves system efficiency.
- Reduce the data redundancy.
- Datasets can be physically distributed over various volumes based on key ranges.
- VSAM datasets can be shared across systems and regions.
- VSAM files can't be displayed and edited in ISPF, and it requires a tool to do the same in ISPF.
- It only works with data stored on a DISK and can't handle data stored on other devices like tapes, etc.
- VSAM only provides security at the dataset level. If any changes are required to the security level, the user should handle them manually.