Ch08 VSAM Slides

8/3/2019 Ch08 VSAM Slides

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VSAM conceptsand terminology


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Virtual Storage Access MethodAccess 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 invirtual storage


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Terminology

Files or data sets familiar concepts

VSAM files referred to a a cluster

Need to DEFINE CLUSTER when definingVSAM files

Requires no special COBOL coding

Except for little used VSAM sequentialfiles


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Virtual Storage Access Method

VSAM Dataset Types

Entry-sequenced ESDSStored in order written may/not be

ordered by a key fieldNon-VSAM sequential datasets just asefficient

ASSIGN clause requires AS-ddname


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Virtual Storage Access Method

VSAM- Dataset Types

Key-sequenced KSDSConsists 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

RRDSRecords accessed by relative record numbers

Can be accessed Sequentially or Randomly


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An employee file with Entry-sequenced (ESDS)organization


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An employee file with key-sequenced (KSDS ) organization


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Relative Record Data Set (RRDS)


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VSAM Record Management

Integrated Catalog Facility ICF

Used to keep track of files on thesystem

There is one Master Catalog

Can be many User Catalogs

Access Method Services general

purpose VSAM file managementprogram

IDCAMS


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VSAM Record ManagementControl Intervals Used to store in fixed length blocks of Logical

records All physical I/O is carried out at the Control

Interval levelControl Intervals exist within ControlAreas CA is usually one cylinder on the disk

Free space can be allocated to both CAand CI VSAM uses the free space when adding new

records


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ICF - The relationships among the master catalog,

user catalogs, and data sets


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VSAM datasets are referred to a clusters

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


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The structure of a control interval

for fixed lengthrecords 10 bytes is reserved for Control Information


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Match your record size to valid sizes for controlintervals that will maximize storage efficiency

Increments of 512 Increments of 2048

(Up to 8192 bytes) (Over 8192 bytes)512 4608 10240 26624

1024 5120 12288 286721536 5632 14336 307202048 6144 16384 327682560 6656 184323072 7168 204803584 7680 225284096 8192 24576

Assume a record size of 300 bytes300 / 4086 = 13 records 3900/4086 = 95%

Control Interval Size


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Record organization

in an ESDS


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Components of a key-sequenced data set


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Accessing a record in a key-sequenced data set


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Free spacedistribution in thedata component ofa KSDS

Need to Insertrecord 6047


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Free space used toinsert a record in

the data componentof a KSDS

Need to insertrecord 6046 ??


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Free space used to

insert a record inthe data component

of a KSDS

VSAM will split thecontrol area ifneeded

Records are alwayskept in physicalsequence


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VSAM supports alternate indices with unique keys


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VSAM supports alternate indices with non-uniquekeys


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Next Time

Access Method Services


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Documents

Ch08 VSAM Slides