VSAM Introduction

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History

  • Before VSAM, older access methods like ISAM and BDAM were used, but they had limitations in handling large and dynamic datasets. VSAM was introduced to overcome these issues by providing better performance, flexibility, and efficient data organization.
  • VSAM was designed to support virtual storage (memory management) introduced in OS/VS1 and OS/VS2, making data access faster and more efficient.
  • VSAM was introduced by IBM in the early 1970s and supports three types of dataset organizations—sequential, indexed, and direct access. Since then, it has been continuously improved to enhance performance and flexibility.

What is VSAM?

  • Virtual Storage Access Method (VSAM) is a file storage and access method used to store and access data efficiently. It represents both the type of dataset and the method used to access and manage that data.
  • VSAM is a record-based system, meaning data is stored as records and can be accessed sequentially, directly, or using keys (index-based).
  • It works with the data that stores only on direct access storage devices (DASDs). i.e., disks.
  • VSAM datasets cannot be directly viewed or edited in ISPF; special tools like DITTO are typically used to browse or modify VSAM data.

Access Methods and File Organization

An access method is system-provided, reusable (re-entrant) component of z/OS. It handles I/O operations, so applications don’t need to deal with low-level data movement between memory and disk.

Dataset types are classified based on the way of organizing the data on IBM disks which is also called as the file organization. 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)

VSAM provides a functional equivalent datasets for most of the existing dataset types as follows -

  • ESDS is the functional equivalent of Physical Sequential organization (PS file).
  • KSDS is the functional equivalent of Indexed Sequential organization (IS file).
  • RRDS is the functional equivalent of Direct Access organization (DA).

VSAM Dataset Types

VSAM record storage format is unique and not understandable by other access methods. Based on the VSAM storage format, accessing, and organizing the records, the datasets are classified into four different types, and those are -

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 -

  • Sequential
  • Direct
  • Dynamic or Skip sequential

VSAM also provides the below access options -

  • Two direct access modes - Addressed or Keyed
  • Two access entities - Logical records or Control intervals
  • Two access directions - Forward or Backward
  • Retrieval options - Using generic key or Using key greater-than-or-equal

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.

Advantages

  • 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.
  • Inserting records is easy when compared to the other access methods.
  • Records can be deleted physically and can reuse the deleted records space to store newly inserted records without reorganization. It helps for faster retrieving.
  • Supports alternate index.
  • Improves system efficiency.
  • Reduce the data redundancy.
  • Datasets can be physically distributed over multiple volumes based on key ranges.
  • Independent of storage device types.
  • VSAM datasets can be shared across systems and regions.
  • A JCL requires a small amount of VSAM dataset information to process. But not detailed information because the VSAM dataset information is stored centrally (VSAM Catalog).
  • The index is small hence uses less memory to store it. Because it uses a key compression algorithm.
  • Provides data security with password protection of dataset at different levels (read, write, update, delete, etc.).

Disadvantages

  • VSAM files data can't be accessed directly in ISPF, and it requires a tool to do the same.
  • VSAM requires more DASD space.
  • It only works with data stored on a DISK and can't handle data stored on other devices like tapes, etc.
  • VSAM has minimal support for data sharing across regions.
  • VSAM only provides security at the dataset level. If any changes are required to the security level, the user should handle them manually.
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