Click to add text Introduction of z/OS Basics © 2006 IBM Corporation Chapter 5: Working with data...

Introduction of z/OS Basics

© 2006 IBM Corporation

Chapter 5: Working with data sets

Chapter 05 Working with Datasets


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Chapter objectives

Be able to:

Explain what a data set is

Describe data set naming conventions and record formats

List some access methods for managing data and programs

Explain what catalogs and VTOCs are used for

Be able to create, delete, and modify data sets



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Key terms in this chapter

block size

catalog

data set

high level qualifier (HLQ)

library

logical record length (LRECL)

member

PDS and PDSE

record format (RECFM)

system managed storage (SMS)

virtual storage access method (VSAM)

VTOC



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What is a data set?

A data set is a collection of logically related data records stored on one disk storage volume or a set of volumes.

A data set can be:– a source program– a library of macros– a file of data records used by a processing program.

You can print a data set or display it on a terminal. The logical record is the basic unit of information used by a program running on z/OS.



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Dataset Naming



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What an access method is

Defines the technique used to store and retrieve data.

Includes system-provided programs and utilities to define and process data sets.

Commonly used access methods include the following:

– VSAM, QSAM, BSAM, BDAM, and BPAM.



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DASD: Use and terminology

Direct Access Storage Device (DASD) is another name for a disk drive.

DASD volumes are used for storing data and executable programs.

Data sets in a z/OS system are organized on DASD volumes.– A disk drive contains cylinders– Cylinders contain tracks– Tracks contain data records.



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Datasets



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Using a data set

To use a data set, you first allocate it. Then, access the data using macros for the access method that you have chosen.

Various ways to allocate a data set:– ISPF data set panel, option 3.2– Access Method Services– TSO ALLOCATE command – job control language (JCL)



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Allocating space on DASD volumes

How space is specified:– explicitly (SPACE parameter)– implicitly (SMS data class)

Logical records and blocks:– Smallest amount of data to be processed– Grouped in physical records named blocks

Data set extents:

– Space for a disk data set is assigned in extents



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Data set record formats

record

block block

block

record record record

record

recordrecord recordrecord record record

block

record record

record

record

record

record record

record

record

recordrecord

BDW

F

FB

V

VB

U

Fixed records.

Fixed blocked records. BLKSIZE = n * LRECL

RDWVariable records.

Variable blocked records. BLKSIZE >= 4 + n * largest LRECL

Undefined records. No defined internal structure for access method.

Record and block descriptors words are each 4 bytes long



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Types of data sets

We discuss three types in this class:

– Sequential, partitioned, and VSAM

A sequential data set is a collection of records written and read in sequential order from beginning to end.

A partitioned data set (PDS) is a collection of sequential data sets, called members.

– Consists of a directory and one or more members.

– Also called a library.

A PDSE is a partitioned data set extended.



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Types of Non-VSAM datasets



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PDS versus PDSE

PDS data sets:

– Simple and efficient way to organize related groups of sequential files.

PDSE data sets:

– Similar to a PDS, but advantages include:

• Space reclaimed automatically when a member is deleted

• Flexible size• Can be shared• Faster directory searches



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What is a data set, and how is it stored

Record 1 Record 2 Record 3 Record 4etc ...

Sequential Data Set

Available space

Partitioned Data Set

Previously used space recoverable by compress utility

Directory

Entry for COMPJCL Entry for JCOPY Entry for SORT1

COMPJCL

JCOPY

SORT1

DASD

Partitionedand

Sequential



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How data is stored in a z/OS system

Data is stored on a disk (DASD), magnetic tape volume, or storage device optical media.

You can store and retrieve records either directly or sequentially.

You use DASD volumes for storing data and executable programs, including the operating system itself, and for temporary working storage.

You can use one DASD volume for many different data sets, and reallocate or reuse space on the volume.



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General Dataset Specifications



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How data sets are named Data set naming convention--Unique name

• Maximum 44 characters– Maximum of 22 name segments: level qualifier

• The first name in the left: high level qualifier (HLQ) Level qualifiers are separated by '.'

– Each level qualifier:• From 1 up to 8 characters• The first must be alphabetical (A-Z) or (@ # $)• The 7 remaining: alphabetical, national, numeric (0-9) or

hyphen (-)• Upper case only

– Example: MYID.JCL.FILE2 HLQ: MYID 3 qualifiers

Member name of PDS (partitioned data set)– 8 bytes long– First byte: alphabetical (A-Z) or special (@ # $)– The 7 remaining: alphabetical, special, numeric (0-9)



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Catalogs and VTOCs

z/OS uses a catalog and a volume table of contents (VTOC) on each DASD volume to manage the storage and placement of data sets.

VTOC:

– Lists the data sets on a volume

– Lists the free space on the volume.



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Volume Table of Contents (VTOC)



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VTOC location

LABEL(volser)

Extents

tracks

free spaceYOUR.DATAMY.DATA

trackstracks

VTOC



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How a catalog is used

A catalog associates a data set with the volume on which the data set is located.

Locating a data set requires:– Data set name– Volume name– Unit I.e.(volume device type e.g. disk, tape)

Typical z/OS system includes a master catalog and numerous user catalogs.



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Catalog Structure

IBMUSER.A1USER.A1SYS1.A1

Catalog Structure

SYSTEM.MASTER.CATALOG

Data Set-SYS1.A1or

HLQs (alias)IBMUSER...USER

Master Catalog

USERCAT.COMPANY

Data Set withHLQ=USER

User Catalog

USERCAT.IBM

User Catalog

Data Set withHLQ=IBMUSER

volume (wrk002)unit (3390)

IBMUSER.A2IBMUSER.A3

volume (wrk001)unit (3390)

volume (012345)unit (tape)

USER.TAPE.A1



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Locating a dataset in MVS



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Catalog and Uncataloged Datasets

Note the ‘ // ‘ and parm statements used for Job Control Language



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Dataset Control Blocks (DSCB)



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VTOC Index Structure

ISPF option 3.4



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Traditional Disk Capacity (DASD or DISK )



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Data management in z/OS Data management involves all of the following tasks:

– allocation,

– placement,

– monitoring,

– migration,

– backup,

– recall,

– recovery, and

– deletion.

Storage management is either manual or automated (or a combination ).

In z/OS, DFSMS is used to automate storage management for data sets.



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Data Facility Subsystem Managed Storage (DFSMS)



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VSAM

VSAM is Virtual Storage Access Method

VSAM provides more complex functions than other disk access methods

VSAM record formats:– Key Sequence Data Set (KSDS)– Entry Sequence Data Set (ESDS)– Relative Record Data Set (RRDS)– Linear Data Set (LDS)



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VSAM - Structure

>VSAM data sets consist of control intervals (CI) and control areas (CA).

>The size of the CI and CA is normally determined by the access method (They are not visible to the user).

>There will be a fixed number of control intervals in each control area. (e.g. CI is a block, CA is a track)

>A control interval normally contains multiple records.

>The records are stored within the control interval at one end

> Control information is at the other end of the control interval

> The space between is the free space.



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Simple VSAM control interval

R1 R2 R3 free space in CIRDF

RDF

RDF

CIDF

Record Descriptor Fields



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VSAM – Structure (cont)

The control information has two types of entry:

- a control interval descriptor field (CIDF) which is always present,

- and record descriptor fields (RDF) which are present when there are records and describe the length of the associated record.

-Free space within a CI is always contiguous.



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VSAM – Structure (cont)

Record insertion

- placed in the correct order relative to other records.

- records may have to be moved in the control interval (CI).

- If there is not enough space, the CI is split (about equally

from a free pool in the Control Area (CA)

- If no more FREE CI in the CA, the CA is split (about

equally)

Record deletion-

- When a record is deleted, later records are moved down so

that the free space remains contiguous.



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VSAM Index Structure



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VSAM – B+ Index

.



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VSAM Keyed Sequential Dataset (KSDS)



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VSAM – Entry Sequenced Data Set (ESDS)

>Records in ESDS are stored in order in which they are written

>Records are loaded irrespective of their contents and their byte addresses cannot be changed.

>An ESDS may have an index defined to it to enable access via keys, by defining an Alternate Index

> Records are accessed based on their sequential order, that is, the order in which they were written to the file;

> accessing a particular record involves searching all the records sequentially until it is located,

or by using a relative physical address (Relative byte address, RBA), i.e. the number of bytes from the beginning of the file to start reading.



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VSAM Sequential Dataset = ESDS



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VSAM – Relative Record Data Set (RRDS)

>A Relative Record Data Set (RRDS) is a type of data set organization used by the VSAM computer data storage system.

> Records are accessed based on their ordinal position in the file (relative record number, RRN).

For example, the desired record to be accessed might be the 42nd record in the file out of 999 total.

>The concept of RRDS is similar to ESDS method, but it can access data in random or dynamic access.

>An RRDS may have an index defined to it to enable access via keys, by defining an Alternate Index



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VSAM - RRDS



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VSAM – Linear Data Set

>A linear data set contains data that can be accessed as byte-addressable strings in virtual storage.

> It is a VSAM data set with a CI size multiple of 4096 bytes.

>An LDS has no embedded control information in its CI

>All LDS bytes are data bytes.

> Logical records must be blocked and deblocked by the program. i.e. Logical records are not apparent from VSAM’s point of view.

In a sense, an LDS is a non-VSAM data set with some VSAM facilities, such as VSAM specific information in the catalog.

>The most common LDS user is IBM DB2®. LDS is the VSAM data set organization that is used by Data-in-Virtual (DIV) facility.



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VSAM LDS – e.g. image



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Basic Parms for VSAM dataset



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z/OS UNIX file systems

z/OS UNIX System Services (z/OS UNIX) allows z/OS to access UNIX files.

A z/OS UNIX file system is hierarchical and byte-oriented.

Files in the UNIX file system are sequential files and accessed as byte streams.

UNIX files and traditional z/OS data sets can reside on the same DASD volume.



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MVS and File Systems..Unix file system as a dataset



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UNIX file system structure

Directory

Directory Directory

Directory Directory

File

File

File

File

File

File

File

File

Directory

File

File

File

File

File

File

File



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Organization of the File System



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Comparison of MVS Datasets and UNIX HFS (Hierarchical file system)



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File System Attributes



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Summary

A data set is a collection of logically related data (programs or files)

Data sets are stored on disk drives (DASD) and tape.

Most z/OS data processing is record-oriented.

Byte stream files are present only in z/OS UNIX.

z/OS records follow well-defined formats, based on record format (RECFM), logical record length (LRECL), and the maximum block size (BLKSIZE).

z/OS data set names have up to 44 characters, divided by periods into qualifiers.



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Summary (continued)

Catalogs are used to locate data sets.

VSAM is an access method that provides more complex functions than other disk access methods.

z/OS libraries are known as partitioned data sets (PDS or PDSE) and contain members.

A file in the hierarchical file system can be either a text file or a binary file.

z/OS treats an entire UNIX file system hierarchy as a collection of “data sets” where

Each “UNIX” data set is a MOUNTABLE file system.

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