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Virtual Storage Access Method. Access Method Interface between application program and Operating System Facilitates data access VSAM Most widely used access method on IBM mainframes Designed to operate in a virtual storage environment Handles all I/O requests to Operating System

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virtual storage access method
Virtual Storage Access Method
  • Access Method
    • Interface between application program and Operating System
    • Facilitates data access
  • VSAM
    • Most widely used access method on IBM mainframes
    • Designed to operate in a virtual storage environment
    • Handles all I/O requests to Operating System
    • No physical I/O is done if record is all ready in virtual storage
terminology
Terminology
  • Files or data sets familiar concepts
  • VSAM files referred to a a cluster
    • Need to DEFINE CLUSTER when defining VSAM files
  • Requires no special COBOL coding
    • Except for little used VSAM sequential files
virtual storage access method vsam dataset types
Virtual Storage Access Method – VSAM – Dataset Types
  • Entry-sequenced – ESDS
      • Stored in order written may/not be ordered by a key field
      • Non-VSAM sequential datasets just as efficient
      • ASSIGN clause requires AS-ddname
virtual storage access method vsam dataset types5
Virtual Storage Access Method – VSAM- Dataset Types
  • Key-sequenced – KSDS
      • Consists of two parts
        • Index – containing key and pointer to record location
        • Data – records stored in primary key sequence
      • Can be accessed Sequentially or Randomly
      • Significant improvement over legacy ISAM files
  • Relative-record – RRDS
      • Records accessed by relative record numbers
      • Can be accessed Sequentially or Randomly
vsam record management
VSAM Record Management
  • Integrated Catalog Facility – ICF
    • Used to keep track of files on the system
    • There is one Master Catalog
    • Can be many User Catalogs
    • Access Method Services – general purpose VSAM file management program
      • IDCAMS
vsam record management10
VSAM Record Management
  • Control Intervals
    • Used to store in fixed length blocks of Logical records
    • All physical I/O is carried out at the Control Interval level
  • Control Intervals exist within Control Areas
    • CA is usually one cylinder on the disk
  • Free space can be allocated to both CA and CI
    • VSAM uses the free space when adding new records
slide12

VSAM datasets are referred to a clusters

Records are grouped together in control intervals, and control intervals are grouped together in control areas

slide14

The structure of a control interval – for fixed length records 10 bytes is reserved for Control Information

control interval size
Control Interval Size

Match your record size to valid sizes for control intervals – that will maximize storage efficiency

slide21

Free space used to insert a record in the data component of a KSDS

VSAM will split the control area if needed

Records are always kept in physical sequence

next time

Next Time

Access Method Services