MVS/ESA Fundamentals of VSAM © Copyright IBM Corp., 2000, 2004. All rights reserved.

MVS/ESA Fundamentals of VSAM

© Copyright IBM Corp., 2000, 2004. All rights reserved.



UNIT

Foundation Concepts

VSAM Data Set Types

VSAM Terminology

Access Method Services

Topics:

VSAM Concepts and Terms



What is an access method?

Most computer applications are designed to manipulate data and generate results based on the data. Data must be stored in a way that its retrieval is easy and quick.

Access methods are ways to maximize the efficiency of data storage and retrieval.

Access Methods

An access method acts as an interface between a processing program and the operating system. Continued…

Unit: VSAM Concepts and Terms

Concepts

Access Methods

DASD

Topic: Foundation Concepts



Virtual Storage Access Method

Virtual Storage Access Method (VSAM) is the first access method specifically designed to operate in a virtual storage environment.

VSAM is more compatible with operating systems MVS, MVS/XA and MVS/ESA than other access methods.

The compatibility of VSAM with various operating systems provides greater efficiency for the application programs using VSAM and also for the overall environment.

Continued…


Concepts

VSAM

DASD




VSAM retrieves a record by following these steps:

1. VSAM interprets the processing program’s

logical request and determines what services are desired.

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

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

4. VSAM locates and extracts the desired data before returning it to the processing program.

Virtual Storage Access Method (cont’d)

Continued…


Concepts

Operating System

DASD

VSAM

Processing Program




Two important variations in the process of retrieving a record are:

• A record requested by a processing program may already be in virtual storage. No physical I/O operations are needed in such a case.

• Sometimes due to the way VSAM stores data and variety of processing options it supports, it must issue several I/O requests to retrieve a single record.


Continued…


Concepts




VSAM groups individual data records into larger units in order to reduce the number of I/O requests required when sequentially retrieving records.

These larger units are transferred between the Direct Access Storage Device (DASD) and virtual storage by the operating system.


Continued…


Concepts

record record record

recordrecord record

DASD




What is the role of VSAM?

VSAM acts as an interface between processing programs and the operating system.

A processing program invokes VSAM routines as subroutines.

In Assembler Language programs, subroutines are invoked by issuing VSAM macros. In High Level Languages, the language compilers convert I/O statements into calls to the appropriate VSAM routines. When the I/O request has been processed, control is returned to the processing program.



Concepts




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)

Overview of Data Set Types


Concepts

Topic: VSAM Data Set Types



Records in an ESDS are stored in the order in which they are written and are retrieved by addressed access. Records are loaded irrespective of their contents and their byte addresses cannot be changed.

ESDS is also referred to as a sequential VSAM data set. This is because records in an ESDS are normally processed sequentially.

ESDS is best suited for applications where most processing is done sequentially.

Entry-Sequenced Data Set


Concepts




Records in a KSDS are stored in key sequence and are controlled by an index. The key field of records determine the order in which records are stored.

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

The advantages of KSDS are:• Sequential processing is useful for retrieving records in the sorted form• Random or direct processing of records is useful in on-line applications

Key-Sequenced Data Set


Concepts




Relative Record Data Set

Records in an RRDS are loaded into fixed-length or

variable length slots. These records are represented

by the Relative Record Numbers (RRNs) of their

slots.

A processing program uses RRN to provide random

access to records.

The records in an RRDS can also be accessed sequentially in its RRN order. It is also possible to convert key values into RRNs.

Accessing data using RRNs is preferred by many applications, such as inventory file management.

Continued…


Concepts

R1 R2 R3

Slots

1 2 3 4

Relative Record Numbers




Linear Data Set

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

An LDS is divided into blocks. These blocks can be sequentially retrieved by a processing program in physical order. A processing program can group several logical records together into a single block

LDS can be kept permanently in store for enhanced performance.

The purpose of the LDS data set type is to provide a format that can be used more easily with the extended addressing feature found in Multiple Virtual Storage / Enterprise Systems Architecture (MVS/ESA).

It is used for special applications requiring large amount of data to be maintained in virtual storage.

Application programs prefer an ESDS to an LDS. ESDS provides same processing abilities and can block and unblock logical records automatically.

Continued…


Concepts




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

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

A KSDS cluster has two data sets. One data set holds the actual data records. The other data set contains an index component.

The index component permits the direct retrieval of data.

What is a Cluster?

Continued…


Concepts

KSDS.CLUSTER

KSDS.INDEX

KSDS.DATA

Topic: VSAM Terminology



What is a control interval?

A control interval is the amount of data transferred between the device and virtual storage.

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

Control Interval

Continued…


Concepts




A control interval is the amount of data transferred between the device and virtual storage.

VSAM groups individual data records into larger units of storage. These units of storage are called control intervals.

All control intervals for a given data set are of the same size. However, the records within a control interval can vary in length.

Control Interval (cont’d)


Concepts

R1 R1 R1Unused Space

420

340

220

9 18 00 5 5

A 2k (2048 bytes) Control Interval




The control intervals for a data set are grouped Into one or more control areas.

The number of control intervals in a control area is fixed by VSAM.

Control Area


Concepts

05 08 09 10

12 14 15 20

21

29

22

30

24 25

35 36

Control Area with Control Intervals




What is Access Method Services?

VSAM uses certain utility programs for managing and maintaining data sets.

Access Method Services (AMS) is a utility that defines VSAM data sets and allocates space for them. It also converts indexed sequential data sets to KSDS with indexes.

AMS can also be used to perform some functions for non-VSAM data sets.

Access Method Services

Continued…

Unit: VSAM Concepts and Terms Topic: Access Method Services

Concepts



What is IntegrateD Catalog Access Method Services?

IntegrateD Catalog Access Method Services (IDCAMS) is the program name for VSAM’s AMS utility. IDCAMS is easy to use and multipurpose in nature.

IDCAMS performs the following functions:

Access Method Services (cont’d)

• It defines a data set

• It loads a data set

• It copies or backs up a data set

• It prints the contents of a data set

• It changes certain attributes of a data set

• It lists data set attributes and statistical information

• It deletes a data set


Concepts



Some of the non-VSAM utility programs are:

• IEBGENER– Used to copy a SAM data set

• IEBCOPY – Used to copy a partitioned data set

Non-VSAM Utility Programs


Concepts



What is defining a data set?

The process of creating catalog entries for a VSAM data set and allocating space for them is called defining the data set.

A data set must be defined before it is loaded with data or accessed by a processing program.

VSAM data sets can be defined using either IDCAMS or JCL.

Defining a VSAM Data Set with IDCAMS

Continued…


Concepts

DASD



The DEFINE CLUSTER function of IDCAMS is used to give the cluster a name and describe its characteristics.

Listed below are some of the characteristics that can be specified during a DEFINE CLUSTER operation:

Defining a VSAM Data Set with IDCAMS (cont’d)

• Cluster name

• Names of the data and index components

• Volume(s) on which the data set is to reside

• Space requirements of the data set

• Record size

• Position and length of the key field for KSDS

• Retention period

• Data set passwords

• Data set type


Concepts



Passwords are implemented to control access to data sets and other system resources.

When a data set is defined, up to four passwords, each representing a different level of protection, can be defined.

You can assign one password to control read access. Another password can be assigned for update access.

Passwords


Concepts

Password

DASD

VSAM

Program

Update Read



Once a data set has been defined, it needs to be loaded with data. The IDCAMS REPRO function is used for initial loading of a data set.

The REPRO function can be used to:

• Load a VSAM data set from a SAM, ISAM or another VSAM data set

• Copy a VSAM data set to a SAM or VSAM but not to an ISAM data set

Initial Loading


Concepts



IDCAMS can be used to perform a variety of data set management functions.

Listed below are some of the commonly used IDCAMS functions:

• PRINT – Prints a VSAM data set

• ALTER – Changes certain attributes of a VSAM data set

• LISTCAT – Lists data set attribute and statistical information

• DELETE – Deletes a data set

Other IDCAMS functions


Concepts



The advantages of VSAM are:

• VSAM supports more data set types• Simplifies record processing• Supports a variety of I/O techniques• Provides greater efficiency for the application programs and for the overall

environment

The major limitation of VSAM is:

• Its data sets must reside on DASD. They cannot be created on tape.

Advantages and Limitations of VSAM


Concepts



Unit: Key-Sequenced Data Sets Topic: KSDS Organization and Storage

Concepts

Index

Data

Highest Level Index Record

Continued…

The index component of a KSDS has the following features:

• The index component is divided into control intervals• Each index control interval is treated as a single record that is divided into

index entries• The index records resemble a tree structure with a single index record at the

highest level

The Index Component



Unit: Key-Sequenced Data Sets Topic: KSDS Organization and Storage

Concepts

Index

Data

Index Set

Sequence Set

The Index Component (cont’d)

There are two types of index records. They are:

• Index Set: It consists of the records in the highest level of the index• Sequence Set: It consists of the records in the lowest level of the index



Continued…

Concepts

Unit: Key-Sequenced Data Sets Topic: KSDS Processing Options

17 26 36 50 54 66 74 99

08 09 11 14

18 19 25 26

27 30 31 33

37 40 43 47

51 52 53 54

55 56 57 59

67 69 70 73

75 84 88 90

Current Record

Current Index Entry

Consider a KSDS shown in the above diagram, which is being processed sequentially.

Assume that the processing program has read all the records up to the record with key 37. The indicated sequence set record and data control interval would currently be in virtual storage.

In response to the next sequential read request, VSAM will return the record at the current position. In this case, it would return the record with key 40.

Sequential Access of Records (cont’d)



Continued…

Concepts

Unit: Key-Sequenced Data Sets

17 26 36 50 54 66 74 99

08 09 11 14

18 19 25 26

27 30 31 33

37 40 43 47

51 52 53 54

55 56 57 59

67 69 70 73

75 84 88 90

Current Record

Current Index Entry

Once the last record in a control interval has been returned to the processing program, VSAM uses the next entry in the sequence set record to retrieve the next control interval from the present control area.

Once the last control interval in the control area has been processed, VSAM uses the horizontal pointer in the sequence set record to retrieve the next sequence set record.

Sequential Access of Records (cont’d)

Topic: KSDS Processing Options



Continued…

Concepts


08

50 99

26 50 66 99

17 26 36 50 54 66 74 99

09 11 14 27 30 31 33 51 52 53 54 67 69 70 73

18 19 25 26 37 40 43 47 55 56 57 59 75 84 88 90

Searching for a record with a key value of 53

A KSDS can process records randomly by supplying the key of the desired records.

Consider the KSDS index and data components illustrated in the above diagram.

Random Access of Records




Continued…

Concepts


08

50 99

26 50 66 99

17 26 36 50 54 66 74 99

09 11 14 27 30 31 33 51 52 53 54 67 69 70 73

18 19 25 26 37 40 43 47 55 56 57 59 75 84 88 90


Suppose the record key supplied by the processing program is 53. Each time a random request is issued, VSAM begins by retrieving the highest level index record and searching through its entries.

Random Access of Records (cont’d)




Continued…

Concepts


08

50 99

26 50 66 99

17 26 36 50 54 66 74 99

09 11 14 27 30 31 33 51 52 53 54 67 69 70 73

18 19 25 26 37 40 43 47 55 56 57 59 75 84 88 90


VSAM examines the key value in each entry until it finds a key which is greater than or equal to the desired key. In this example, VSAM will stop at the second entry in the highest level index record (53 is greater than 50 but less than 99).






Continued…

Concepts


08

50 99

26 50 66 99

17 26 36 50 54 66 74 99

09 11 14 27 30 31 33 51 52 53 54 67 69 70 73

18 19 25 26 37 40 43 47 55 56 57 59 75 84 88 90


Using the corresponding pointer, the next lower level index record is retrieved. This record is processed until a key which is greater than or equal to the desired key is found.





Continued…

Concepts


08

50 99

26 50 66 99

17 26 36 50 54 66 74 99

09 11 14 27 30 31 33 51 52 53 54 67 69 70 73

18 19 25 26 37 40 43 47 55 56 57 59 75 84 88 90


Eventually, the search will lead to a sequence set record. Within the record, VSAM locates the first key having a key value greater than or equal to the key value of the desired record.





Continued…

Concepts


08

50 99

26 50 66 99

17 26 36 50 54 66 74 99

09 11 14 27 30 31 33 51 52 53 54 67 69 70 73

18 19 25 26 37 40 43 47 55 56 57 59 75 84 88 90


The pointer in this entry is used to retrieve a data control interval. VSAM searches the control interval sequentially, for the desired record. If the record is present, VSAM returns it to the processing program. If not, VSAM signals a record-not-found condition.





Continued…

Concepts


08

50 99

26 50 66 99

17 26 36 50 54 66 74 99

09 11 14 27 30 31 33 51 52 53 54 67 69 70 73

18 19 25 26 37 40 43 47 55 56 57 59 75 84 88 90


The pointer in the index records define a path that VSAM follows to locate a record.

The path VSAM would follow in this example is illustrated in the above diagram.




VSAM

377Vert,

Pntr. 619 Vert.Pntr 800

Vert.

Pntr.

280Vert,

Pntr. 327 Vert.Pntr 377

Vert.

Pntr. 469Vert,

Pntr. 619 Vert.Pntr … …. 700

Vert,

Pntr. 800 Vert.Pntr … …

Index Set

Sequence Set Sequence Set Sequence SetHoriz.

Pntr.

Horiz.

Pntr.

627

642

658

675

700

251

269

280

FREE

312

318

327

FREE

345

346

377

FREE

394

400

449

469

FREE

500

502

598

617

619

717

722

746

748

800

FREE FREE

ControlInterval

ControlInterval

ControlInterval

Control Area Control Area Control Area



377Vert,

Pntr.619Vert.Pntr 700

Vert.

Pntr.

280Vert,

Pntr.327Vert.Pntr 377

Vert.

Pntr. 469Vert,

Pntr.598Vert.Pntr 619Vert.

Pntr.

Index Set

Sequence Set Sequence Set Sequence Set

Horiz.

Pntr.

Horiz.

Pntr.

251

269

280

FREE

345

346

377

FREE

394

400

410

449

469

500

502

598

FREE

Control Area Control Area

658Vert,

Pntr.700Vert.Pntr

Vert.Pntr

…

627

642

658

FREE

FREE

Control Area

800Vert.

Pntr.

Sequence Set

730Vert,

Pntr.800Vert.Pntr

Vert.Pntr. …

FREE

Control Area

312

316

318

327

FREE

617

618

619

FREE

675

680

700

FREE

746

748

800

FREE

717

722

730

FREE

Horiz.

Pntr.

Control Interval Split

Control Interval Split

Control Area Split

VSAM

MVS/ESA Fundamentals of VSAM © Copyright IBM Corp., 2000, 2004. All rights reserved.

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Transcript of MVS/ESA Fundamentals of VSAM © Copyright IBM Corp., 2000, 2004. All rights reserved.