VSAM Presentación (en inglés)

7/17/2019 VSAM Presentacin (en ingls)

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MVS/ESA Fundamentals of VSAM

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%&I'

Foundation Con(epts

VSAM )ata Set 'ypes

VSAM 'erminology

A((ess Method Ser#i(es

'opi(s*

VSAM Con(epts and 'erms


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What is an access method?

Most (omputer appli(ations are designed to

manipulate data and generate results

ased on the data. )ata must e stored

in a -ay that its retrie#al is easy and

ui(.

A((ess methods are -ays to ma0imi1e the

effi(ien(y of data storage and retrie#al.

Access Methods

An a((ess method a(ts as an interfa(e

et-een a pro(essing program and the

operating system. Continued2

Unit:VSAM Concepts and Terms

Con(epts

Access Methods

DASD

Topic:Foundation Concepts


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Virtual Storage Access Method

Virtual Storage A((ess Method 3VSAM4 is the

first a((ess method spe(ifi(ally designed to

operate in a #irtual storage en#ironment.

VSAM is more (ompatile -ith operating

systems MVS, MVS/5A and MVS/ESA than

other a((ess methods.

'he (ompatiility of VSAM -ith #arious

operating systems pro#ides greater effi(ien(y

for the appli(ation programs using VSAM and

also for the o#erall en#ironment.

Continued2


Con(epts

VSAM

DASD



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VSAM retrieves a record by following these

steps:

7.VSAM interprets the pro(essing program8s

logi(al reuest and determines -hat

ser#i(es are desired.

.VSAM maes the reuired Input or 9utput

3I/94 reuest3s4 to the operating system.

+.'he operating system performs the

physi(al I/9 operation3s4 et-een thede#i(e and the storage.

".VSAM lo(ates and e0tra(ts the desired

data efore returning it to the pro(essing

program.

Virtual Storage Access Method (contd!

Continued2


Con(epts

"perating System

DASD

VSAM

#rocessing #rogram



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'-o important #ariations in the pro(ess of retrie#ing a re(ord are*

A re(ord reuested y a pro(essing program may already e in #irtual

storage. &o physi(al I/9 operations are needed in su(h a (ase.

Sometimes due to the -ay VSAM stores data and #ariety of pro(essing

options it supports, it must issue se#eral I/9 reuests to retrie#e a single

re(ord.


Continued2


Con(epts



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VSAM groups indi#idual data re(ords into

larger units in order to redu(e the

numer of I/9 reuests reuired -hen

seuentially retrie#ing re(ords.

'hese larger units are transferred et-eenthe )ire(t A((ess Storage )e#i(e

3)AS)4 and #irtual storage y the

operating system.


Continued2


Con(epts

record record record

recordrecord record

DASD



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Copyright IBM Corp., !!!, !!". All rights reser#ed. $age igh =e#el =anguages, the language (ompilers (on#ert I/9 statements into

(alls to the appropriate VSAM routines. ?hen the I/9 reuest has een

pro(essed, (ontrol is returned to the pro(essing program.



Con(epts



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VSAM supports the follo-ing data set types*

EntrySeuen(ed )ata Set 3ES)S4

eySeuen(ed )ata Set 3S)S4

elati#e e(ord )ata Set 3)S4

=inear )ata Set 3=)S4

"verview of Data Set $ypes


Con(epts

Topic:VSAM Data Set Types


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e(ords in an ES)S are stored in the order in -hi(h they are -ritten and are

retrie#ed y addressed a((ess. e(ords are loaded irrespe(ti#e of their (ontents

and their yte addresses (annot e (hanged.

ES)S is also referred to as a seuential VSAM data set. 'his is e(ause re(ords in

an ES)S are normally pro(essed seuentially.

ES)S is est suited for appli(ations -here most pro(essing is done seuentially.

%ntry&Se'uenced Data Set


Con(epts



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e(ords in a S)S are stored in ey seuen(e and are (ontrolled y an inde0. 'he

ey field of re(ords determine the order in -hi(h re(ords are stored.

In a S)S, re(ords (an e pro(essed oth seuentially and randomly using their

ey field #alues.

'he ad#antages of S)S are*

Seuential pro(essing is useful for retrie#ing re(ords in the sorted form

andom or dire(t pro(essing of re(ords is useful in online appli(ations

ey&Se'uenced Data Set


Con(epts



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)elative )ecord Data Set

e(ords in an )S are loaded into fi0edlength or

#ariale length slots. 'hese re(ords are represented

y the elati#e e(ord &umers 3&s4 of their

slots.

A pro(essing program uses & to pro#ide randoma((ess to re(ords.

'he re(ords in an )S (an also e a((essed

seuentially in its & order. It is also possile to

(on#ert ey #alues into &s.

A((essing data using &s is preferred y many

appli(ations, su(h as in#entory file management.

Continued2


Con(epts

)* )+ ),

Slots

7 + "

elati#e e(ord &umers



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-inear Data Set

=)S is a data set (ontaining only a (ontiguous string of data ytes -ith no inter#ening (ontrolinformation.

An =)S is di#ided into lo(s. 'hese lo(s (an e seuentially retrie#ed y a pro(essingprogram in physi(al order. A pro(essing program (an group se#eral logi(al re(ords togetherinto a single lo(

=)S (an e ept permanently in store for enhan(ed performan(e.

'he purpose of the =)S data set type is to pro#ide a format that (an e used more easily -iththe e0tended addressing feature found in Multiple Virtual Storage / Enterprise SystemsAr(hite(ture 3MVS/ESA4.

It is used for spe(ial appli(ations reuiring large amount of data to e maintained in #irtualstorage.

Appli(ation programs prefer an ES)S to an =)S. ES)S pro#ides same pro(essing ailities and(an lo( and unlo( logi(al re(ords automati(ally.

Continued2


Con(epts



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A (luster is the (olle(tion of physi(al

data sets that mae up one logi(al

data set.

'he (on(ept of a (luster is more suited

for a S)S.

A S)S (luster has t-o data sets.

9ne data set holds the a(tual data

re(ords. 'he other data set

(ontains an inde0 (omponent.

'he inde0 (omponent permits the

dire(t retrie#al of data.

What is a .luster?

Continued2


Con(epts

S)S.C=%S'E

S)S.I&)E5

S)S.)A'A

Topic:VSAM Terminology


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What is a control interval?

A (ontrol inter#al is the amount of data transferred et-een the de#i(e and #irtual

storage.

?hen a re(ord is read from or -ritten to a data set, VSAM groups indi#idual data

re(ords into larger units of storage. 'hese units of storage are (alled (ontrol

inter#als.

.ontrol /nterval

Continued2


Con(epts



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A (ontrol inter#al is the amount of data

transferred et-een the de#i(e and

#irtual storage.

VSAM groups indi#idual data re(ords

into larger units of storage. 'heseunits of storage are (alled (ontrol

inter#als.

All (ontrol inter#als for a gi#en data set

are of the same si1e. >o-e#er, the

re(ords -ithin a (ontrol inter#al

(an #ary in length.

.ontrol /nterval (contd!


Con(epts

)* )* )*0nused

Space

1

+

2

,

1

2

+

+

2

3 *

4 2

2 5

5

A 3!"< ytes4

Control Inter#al



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'he (ontrol inter#als for a data set are

grouped Into one or more (ontrol

areas.

'he numer of (ontrol inter#als in a

(ontrol area is fi0ed y VSAM.

.ontrol Area


Con(epts

25 24 23 *2

*+ *1 *5 +2

+*

+3

++

,2

+1 +5

,5 ,6

Control Area

-ith Control Inter#als



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VSAM Presentación (en inglés)