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Mainframe ApplicationProgramming

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Course schedule

UNITVSAM Intro

Access Methods

VSAM Introduction

VSAM Organization

Control Interval

Topics:

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Listed below are some of the traditional access methods thatare available for the Multiple Virtual Storage

(MVS) environment:

Queued Sequential Access Method (QSAM)

Basic Sequential Access Method (BSAM)

Indexed Sequential Access Method (ISAM)

Basic Direct Access Method (BDAM)

Partitioned Data Set Extended (PDS-E)

Access Methods (contd)

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VSAM Introduction

VSAM stands for Virtual Storage Access Method

Used to access the data sets quickly and effectively.

Used to organize, store, catalog, retrieve and delete data sets.

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ROLE OF VSAM

VSAM acts as an interface between processing programs

and the operating system.

VSAM groups individual data records into larger units inorder to reduce the number of I/O requests required when

sequentially retrieving records.

These larger units are transferred between the DirectAccess Storage Device (DASD) and virtual storage by

the operating system.

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How does VSAM retrieve a record

In retrieving a record VSAM goes throughthe following steps:

1. VSAM interprets the processing programs

logical request and determines whatservices are desired.

2. VSAM makes the required Input or Output(I/O) request(s) to the operating system.

3. The operating system performs thephysical I/O operation(s) between thedevice and the storage.

4. VSAM locates and extracts the desireddata before returning it to the processing

program.

Operating System

DASD

VSAM

Processing Program

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ADVANTAGES OF VSAM

Data can be accessed faster.

Records can be inserted in more efficient manner.

Deletion of records results in them being physically removedfrom the disk.

Records can be accessed sequentially or randomly.

VSAM data sets are device independent.

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DISADVANTAGES OF VSAM

VSAM data sets require more storage space than

other types of data sets.

VSAM data set require additional free space that

must be embedded in them.

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DATA SETS IN VSAM

VSAM supports the following data set types:

Entry-Sequenced Data Set (ESDS)

Key-Sequenced Data Set (KSDS)

Relative Record Data Set (RRDS)

Linear Data Set (LDS)

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Entry-Sequenced Data Set

Records in an ESDS are stored in the order inwhich they are written and are retrieved byaddressed access.

Records are loaded irrespective of their contentsand their byte addresses cannot be changed.

ESDS is also referred to as a sequential VSAMdata set. This is because records in an ESDS are

normally processed sequentially.

ESDS is best suited for applications where mostprocessing is done sequentially.

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Key-Sequenced Data Set

Records in a KSDS are stored in key sequence and arecontrolled by an index, determine the order in which recordsare stored.

In a KSDS, records can be processed both sequentially andrandomly using their key field values.

The advantages of KSDS are:

Sequential processing is useful for retrieving records inthe sorted form

Random or direct processing of records is useful in on-line applications

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Relative Record Data Set

Records in an RRDS are loaded into fixed-length or variablelength slots.

These records are representedby the Relative RecordNumbers (RRNs) of their

slots.

A processing program uses RRN to provide random access torecords.

R1 R2 R3

Slots

1 2 3 4

Relative Record Numbers

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Linear Data Set

LDS is a data set containing only a contiguous string of databytes with no intervening control information.

An LDS is divided into blocks. These blocks can be

sequentially retrieved by a processing program in physicalorder.

A processing program can group several logical recordstogether into a single block

LDS can be kept permanently in store for enhancedperformance.

Continued

Concepts

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Relative Byte Address

What is a Relative Byte Address?

The Relative Byte Address (RBA) of a record is its displacement

(in bytes) from the beginning of the data set.

VSAM treats data as a contiguous string of bytes. This approach

makes the address of a record device-independent.

A VSAM data set can be moved without affecting the RBAs of

its records.

Continued

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In addition to data recordsVSAM also stores controlInformation. The presence ofcontrol Information affects theRBAs of subsequent data

records. The example represents a VSAM

data set containing 100-byte,fixed-length records.

The RBA of the first record is 0.

The RBA of the second record is100.

The RBA of the third record is200 and so on.

Relative Byte Address (contd)

Record 1 Record 2 Record3

Relative Byte Addresses

0 100 200 300

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What is a cluster?

A cluster is the collection of physical data sets thatmake up one logical data set.

The concept of a cluster is more suited for a KSDS.

Cluster

Continued

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A KSDS cluster has two datasets.

One data set holds the actual

data records.

The other data set containsan index component.

The index component permitsthe direct retrieval of data.

Cluster (contd)

Continued

KSDS.CLUSTER

KSDS.INDEX

KSDS.DATA

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With an ESDS, an RRDS

and an LDS, the cluster

name and the data set

component name bothrefer to the same data set

and

a cluster consists of only asingle physical data set:

the data component.

Cluster (contd)

Continued

Concepts

A BC

Cluster

Related Data Sets

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What is a control interval?

o A control interval is the amount of data transferred between the

device and virtual storage.

o When a record is read from or written to a data set, VSAMgroups individual data records into larger units of storage. Theseunits of storage are called control intervals.

o Size of a control interval should be minimum 512 bytes andincreased by multiples of 512 upto a limit of 8 K. Beyond that theincrements should be 2KB up to a maximum of 32KB.

o Control interval size = n * 512 or n * 2048 where n is between1 to 16.

R1 R1 R1UnusedSpace

420

340

220

9 18 00 5

5

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control interval has 4 components :

Data

Control interval description field

Record description field

Free space

Data

Contains the actual data processed by the program

Control interval description field

Contains the information about the free space within the CI.

field is with 4 bytes long.

Unused Space (No Records)

Control Interval 1 CIDF

Unused Space (1055)980

1055

420220 220340420340

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Record Description Field:

Contains the information about the records within the data space.

An RDF is a three byte field that is used to define the location and

length of a record or a group of records.

The RDFs immediately precede the CIDF, at the end of the control

interval.

Unused Space (1055)R4432

R3432

R2432

R1432

4324

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In an ESDS records are stored in the order in which they are writtenand can be read in the same order.

The characteristics of ESDS are summarized below:

Records are stored sequentially.

Records can be of fixed or variable length.

Records are physically grouped into control intervals.

Control intervals contain control information along with data.

ESDS Organization

R4R3R2R1 R8R7R6R5 R4R3R2R1

Control Interval 1 Control Interval 2 Control Interval 3

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Jcl for ESDS

.

//SAMP003B JOB ,,CLASS=M,

// MSGLEVEL=(1,1),NOTIFY=SAMP003,TIME=(1)//EXEC1 EXEC PGM=IDCAMS

//SYSPRINT DD SYSOUT=*

//SYSIN DD *DEFINE CLUSTER -

( -

NAME(SAMP003.ESDS.CLUSTER) -

TRACKS(2,1) -

CISZ(512) -

RECORDSIZE(80,80) -NONINDEXED -

)

DATA(NAME(SAMP003.ESDS.DATA)

)

/*

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How is the size of a control interval determined?

The size of a control interval is determined when the data set isdefined.

The size can be defined in either of the following ways:

The size can be defined with the DEFINE CLUSTER commandthat defines the data set.

VSAM selects the size that best utilizes DASD storage.

Control Intervals

Continued



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Control Intervals (contd)

What is control information?

VSAM uses some information to locate records within acontrol interval. This information is called control

information. Data records are stored at the beginning of the control

interval, while the control information is located at the endof the control interval.

Control Info



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CIDF (contd)

What does CIDF contain? The CIDF contains the following two values, each two bytes

long:

The first value indicates where the unused space in the

control interval begins, stored as a displacement from thebeginning of the control interval.

The second value indicates the length of the unused space.

Unused Space (No Records) 0

Control Interval 11020

CIDF

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Record Definition Field

What is a Record Definition Field?

The data records in a control interval are described by a group of

record definition fields (RDFs).

An RDF is a three byte field that is used to define the locationand length of a record or a group of records.

The RDFs immediately precede the CIDF, at the end of the

control interval.


Control Interval 1

1055

R3420

R2340

R1220

220340420

CIDFRDF3 RDF2 RDF1

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Record Definition Field Case 1

Case 1: Consider a case, where no two consecutive records

have the same length and the control interval has unusedspace.


Control Interval 1

1055

CIDF

R3420

R2340

R1220

220340420

RDF3 RDF2 RDF1

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Case 2: Consider a case, where all records in a control interval

are of the same length and the control interval has unused

space.

Unused Space (1055)

Control Interval 1 CIDF

R4432

R3432

R2432

R1432

RDF2 RDF11728

310

4324

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Case 3: Consider a case, where records are of variable

length, but some consecutive records are of same length.

Unused Space 228

1804

Control Interval 1

228

CIDF

R4380

R3380

R2380

3332R1332

RDF4 RDF3

R5332

332380

RDF2 RDF1

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How is space wasted in a control interval?

The example shows how a data set, containing records of3000 bytes and 5000 bytes, would be stored if a 5K (5120bytes) control interval is used.

The following inferences can be made from the givenexample:

Only a single record of either size can fit in a controlinterval

For every 3000-byte record, 2113 bytes are unused andfor every 5000-bytes record, 113 bytes are unused

As a result, just for 5 records, 6565 bytes of space iswasted

Managing Control Intervals

Continued

3000 50003000 30005000

5K (5120 bytes) Control Intervals

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How can you avoid wastage of space?

There are two solutions to prevent space wastage in controlintervals. They are:

Increasing the control interval size

Spanned records

Managing Control Intervals (contd)

Continued

3000 50003000 30005000


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Increasing the Size of Control Intervals

In this case, a 16K control interval might result in a less wasted

space, depending on the proportion and sequence of the 3000-byte and 5000-byte records. However, as VSAM always reads

the CIDF into main storage, this solution is not very efficient.


Continued

16K Control Interval

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Spanned Records The problem of wasted space can be resolved by reducing the

control interval size and directing VSAM to split

records over control intervals. Records which are split betweencontrol intervals are called spanned records.

A spanned record, thus, is a logical record contained in morethan one block.

In the given case, if 3K intervals are used with spanned records,instead of 5K control intervals, the total amount of wastedspace will be reduced to 4234 bytes.



3000 3062 1038 3000 3000 3062 1938

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