📄 VSAM

Introduction to VSAM

Beginner 🕑 20 min read 👁 0 views

What is VSAM?

VSAM (Virtual Storage Access Method) is an IBM file storage access method used on mainframes. It provides high-performance access to data sets with various organization methods.

VSAM File Types

KSDS (Key Sequenced Data Set)
- Records accessed by unique key
- Records stored in key sequence
- Most commonly used type
ESDS (Entry Sequenced Data Set)
- Records stored in arrival order
- Accessed by RBA (Relative Byte Address)
- Like sequential files
RRDS (Relative Record Data Set)
- Records accessed by relative record number
- Fixed-length records only
- Direct access by slot number
LDS (Linear Data Set)
- Byte-stream data
- Used by DB2 and other products

VSAM Components

Cluster - The complete VSAM file
Data Component - Contains actual records
Index Component - Contains key index (KSDS only)
Catalog - Contains cluster definitions

VSAM vs Non-VSAM

Feature	VSAM	Non-VSAM
Organization	KSDS, ESDS, RRDS	PS, PO
Key Access	Yes (KSDS)	No
Catalog	Required	Optional
Utility	IDCAMS	IEBGENER

Code Example

//DEFCLUST EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN    DD *
  DEFINE CLUSTER -
         (NAME(MY.VSAM.KSDS) -
          INDEXED -
          KEYS(6 0) -
          RECORDSIZE(100 100) -
          SHAREOPTIONS(2 3) -
          CYLINDERS(10 5)) -
         DATA -
         (NAME(MY.VSAM.KSDS.DATA)) -
         INDEX -
         (NAME(MY.VSAM.KSDS.INDEX))
/*

//* ESDS DEFINITION
//SYSIN    DD *
  DEFINE CLUSTER -
         (NAME(MY.VSAM.ESDS) -
          NONINDEXED -
          RECORDSIZE(80 80) -
          CYLINDERS(5 2))
/*

//* RRDS DEFINITION
//SYSIN    DD *
  DEFINE CLUSTER -
         (NAME(MY.VSAM.RRDS) -
          NUMBERED -
          RECORDSIZE(100 100) -
          CYLINDERS(5 1))
/*