MVS (Day 2). Copyright © 2005, Infosys Technologies Ltd 2 ER/CORP/CRS/OS01/003 Version No: 1.0...

MVS (Day 2)

Copyright © 2005, Infosys Technologies Ltd

2 ER/CORP/CRS/OS01/003

Version No: 1.0

Agenda

• Data Management– Definition of data set and Record Formats– Describe the organization of data sets – VSAM and Non-VSAM– VSAM Performance Tuning – MVS label processing– Storage Management Subsystem

• Job Management– What is a “job” ?– How does JCL specify its processing requirements ?– Job Entry Subsystem - overall responsibility for job management.– Phases a job goes through as it is processed.– Sample output of messages & results.



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Data sets

• A data set is mainframe term denoting a file -– With in a data set, data is organized into smaller units called records, which can be

processed individually by application programs– MVS keeps track of data through labels, catalogues and different data set

organizations



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VARIABLE BLOCKED

1 LOGICAL1 PHYSICAL

FIXED UNBLOCKED

LOGICAL LOGICAL LOGICAL LOGICALFIXED BLOCKED

4 BBL

4 BRL

RECORDVARIABLE UNBLOCKED

UNDEFINED

Record Formats

BL RL RECORD RL RECORD…



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BLOCK (physical record)

logical recordINTER BLOCK GAP (IBG)

All logical records are the same sizelimit 32,760.

All blocks are the same size (except the last one)limit 32,760.

Fixed Block Format



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BLOCK

LOGICAL RECORD

LOGICAL RECORD

4 4 4 4

LOGICAL RECORD

BLOCK DESCRIPTOR WORD (4 BYTES)

INTER BLOCK GAP

4

RECORD DESCRIPTOR WORD (4 BYTES)

Logical records in a block can have different sizes.The first four bytes define the size of each record. Limit 32,756 bytes.

Blocks can have different sizes.

The first 4 bytes of each block define its size.Limit 32,760 bytes.

Variable Blocked Format



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BLOCK BLOCK BLOCK

INTER BLOCK GAP

Blocks can be of any size up to the limit of 32,760 bytes.Logical records do not exist.

Undefined Format



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VSAM and Non-VSAM

• Two different data management environments :– VSAM

• Virtual storage access method

• required utility called Access Method Services to create VSAM data sets.

• ESA SMS (Storage Management Services) allows creation of VSAM data sets through JCL

– Non-VSAM• Older method

• File can be created by JCL specifications



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Non-VSAM Data Set Organizations

PHYSICALLY SEQUENTIAL

PARTITIONED (PDS)

DIRECT ACCESS

INDEXED SEQUENTIAL

NON-VSAM



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Physical sequential (PS) data sets …

• Records stored one after another in consecutive sequence -– a data element within record can contain key value that’s used to sequence records

• Random access of data not possible

• Suitable for just about any type of I/O device



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PHYSICALLY SEQUENTIAL DATA SETS CAN RESIDE ON ALMOST ANY STORAGE DEVICE

IT’S THE ONLY DATA SET ORGANIZATION WHICH CANEXIST ON MAGENITIC TAPE.

THIS IS THE ORGANIZATION DEFAULT, SO AN ACCESS METHOD NEED NOT BE CODED.

HOWEVER, DSORG=PS, CODED ON A DCB, WILL EXPLICITLYREQUEST AN ACCESS METHOD.

Physical Sequential



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Partitioned data sets (PDS) …

• Divided into one or more members, each of which can be processed as if it were a separate physical sequential file.

• Each members name is stored in a directory.

• Also called a PDS or library.

• Entire library can be processed a unit.

• Widely used by MVS to store system data.



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PDS …



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DEPT01.........................................................LOCATION DEPT02.........................................................LOCATION DEPT03.........................................................LOCATION

DEPT01 DEPT02 DEPT03

ACCOUNTS

PDS …



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ENTRY FORMEMBER A

ENTRY FORMEMBER B

MEMBER A

MEMBER B

PDS …



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MEMBER NAME TTR C OPTIONAL USER DATA

TTRN TTRN TTRN

POINTER TO FIRST RECORD OF MEMBER

38 0-31MAX 62 BYTES

‘1’ IF ALIAS

NO. OF USER DATA TTRN’S

NO. OF USER DATAHALF WORDS

PDS



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Direct data sets

• Each record can be accessed at random based on records’ disk location.

• Isn't commonly used because of programming complexities involved in calculating disk addresses.



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Indexed sequential data sets …

• Records can be accessed sequentially & randomly using a index that relates key field values to locations of corresponding data record.

• Random access of records based on key value possible.



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FIRST NAME LAST NAME EMPLOYEE NUMBER

THOMAS BLUESTONE 00008

WILLIAMS COLLINE 00002

RONALD GARCIA 00010

STANLEY ABBOTT 00001

EMPLOYEE DISK NAME LOCATION

00001 5

00002 2

00008 7

00010 1

INDEX COMPONENT DATA COMPONENT

Indexed Sequential data sets



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VSAM Design Objectives

• Operating System independence

• Device independence

• Data integrity

• Data security

• File performance

Uses of VSAM

• VSAM is the choice for high performance, random access applications.

• VSAM is ideal for CICS, IMS and batch databases.

• VSAM is used for a large part of the data that MVS creates and/or uses.



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VSAM data sets

• Entry-sequenced data set ( ESDS )– Similar to physical sequential files but can reside only on DASDs.

• Key-sequenced data set ( KSDS )– Similar to indexed sequential data set– Most common

• Relative record data set ( RRDS )– Similar to a direct data set– retrieve record by specifying its location relative to start of the file

• Linear Dataset



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VSAM Data Set Organization

ENTRY-SEQUENCED

KEY-SEQUENCED

RELATIVE RECORD

VSAM

LINEAR RECORD



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VSAM Performance Tuning

• Control Interval Size

• Free space

• Buffer space



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MVS label processing

• Labels are special records in data sets used by MVS to identify data sets.

• DASD labels -– Volume labels ( VOL1 label )

• provides a volume-serial ( unique, 6 character )

• address of VTOC

– File labels

• VTOC : Special file that contains file labels of data sets on the volume



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• File labels are also called DSCBs (data set control blocks) , have several formats:

– Format 1 DSCB : gives data set name, etc.,– Format 5 DSCB : describes VTOC

• Space allocated to DASD files in areas called extents - initial primary extent, then one or more secondary extents as file grows ( records are added ).



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DSCB Formats

• Format 1 DSCB : primary extent and three secondary extents

• Format 3 DSCB : thirteen additional extents

• Format 5 DSCB : defines upto 26 free extents



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MVS Catalogs

• Catalogs record location of files so that its volume-serial need not be specified to locate it.

• Master catalogs :– entries that identify system data sets &– entries that identify user catalogs

• User catalogs :– entries that identify user data sets



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MVS Catalogs



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VTOC

ABE.FILE1ABE.FILE2.USER.CATALOG.TST.PSTTST.SEC

ABE.FILE1 3390 111111ABE.TP1 3480 503308ABE.TP2 3480 566637ABE.TP3 3480 503984FILEX.TT 3480 900035TST.LIBA 3390 222222TST.LIBB 3390 222222

DSN UNIT VOLUME

ABE.FILE1

ABE.FILE2

USER CATALOG

TST.PST

TST.SEC

3390 VOLUME 111111

Volume Table of Contents



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A B

A.B A.C A.D

A.C.E A.C.FA.D.G0000V00

A.D.G0001V00

Catalog Structure



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Data set processing by MVS :

• Three distinct steps :– Allocation

• unit ( device ), volume, data set

• DISP parameter qualifies how a non-VSAM file is allocated

– Processing• Access methods used for processing

– basic, queued or VSAM

– open, I/O and close processing– Deallocation

• auto deallocate after job over



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Storage Management subsystem :

• MVS/ESA feature, SMS is a separate software that automates storage management.

• SMS uses storage class, data class etc., to automate storage management

• Enables allocating VSAM data sets using JCL– Without SMS an AMS ( Access Method Service ) utility called IDCAMS

needs to be used for this purpose.



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Job Management



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Job Management

• A job is the execution of one or more related programs in sequence– Each program to be executed is a job step

• Job control language or JCL :– A set of control statements that provide the specifications necessary to process a

job.



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The 3 Basic JCL statements

• JOB– Info that identifies a job

• EXEC– Name of program to be executed

• DD– Defines data, one per file of data referred



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Basic format of a JCL

//SYDOEJ JOB USER=SYSDOE,PASSWORD=xxxxx

Identifier field

Name field

Operation field

Parameters field



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Job that prints a library member

//SYDOEJ JOB USER=SYSDOE,PASSWORD=xxxxxxx// EXEC PGM=IEBGENER//SYSPRINT DD SYSOUT=A//SYSUT1 DD DSN=SYDOE.COPYLIB.COBOL(OPENITEM),// DISP=SHR//SYSUT2 DD SYSOUT=A//SYSIN DD DUMMY

( Note JCL syntax for continuing a statement on the next line. )



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JES - Basic responsibilities

• Enter job into system.

• Decide when job is be processed.

• Pass job to MVS for processing.

• Deliver job’s printed output to correct destination.



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How is an MVS job processed ?(JES)

1.The job is submitted.

2.The job is selected for execution.

3.The job is executed.

4.The job’s output is processed.

5.The job is purged.



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Submitting a job

• Create JCL at terminal using an editor (ISPF).– Job stream now in a file on DASD

• Use SUBMIT command to enter job into system :– JES copies job stream into JES spool ( a special system DASD file ).



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Scheduling for execution …

• Each initiator has one or more job classes associated with it– Can control no. of jobs of each class and combinations there-of.

• Within a class initiators select a job for execution based on priority ( 0 - 15 ).



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• Job class & priority decides when a job is actually scheduled to run.

• Initiator – prog. that runs in sys region of address space eligible for batch jobs

– Examines JES spool, selects job, executes job in address space & returns to spool for next job.

• Each initiator handles one job and no. of initiators can be varied dynamically.



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• A typical Job-class assignment :-

Job Class Characteristics

A Will execute within 15 mins of submissionB 30 minsC 1 hourD Will execute overnightH Will be held until released by operatorL Within 15 mins of submission; each step limited to 1 min execution timeT requires tape processing



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Scheduling for execution

• Typical initiator-class assignment :

Initiator Eligible job classes

1 A2 B,C,D,H,L,T3 B,C,D,H,L,T4 B,C5 B,C6 C



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Executing a job …

• Initiator selects job for exec and invokes interpreter.

• Interpreter examines job stream and creates a series of control blocks which describe all data sets required.

• Then, initiator,– invokes allocation routines– builds user region and executes job in it– invokes unallocation routines after job over.



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Executing a job



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Processing a job’s output

• Like jobs, SYSOUT data is also assigned to different output classes :– Class A : Std printer output, Class Z : held output, etc.,

• Single job may produce SYSOUT data using more than one output class :– Class A for system messages & program output, class D for some of job’s

spooled output.

• Specs for output like no. of copies etc.,



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Purging a Job

• JES spool space held by job freed, after output processing has been done.

• JES control blocks associated with job deleted.



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Alternative data set allocations :

• Job-step allocation :– allocate/deallocate on a step by step basis.

• JES3 allocation :– part of resources pre-allocated by JES3 and rest by MVS during execution.– prevents resource conflicts

• Dynamic allocation : JES2 & JES3– No allocation till request.– Mainly used during TSO sessions.



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Summary

• Data Management– Definition of data set and Record Formats– Describe the organization of data sets – VSAM and Non-VSAM– Performance Tuning– Storage Management Subsystem

• Job Management– What is a “job” ?– How does JCL specify its processing requirements ?– Job Entry Subsystem - overall responsibility for job management.– Phases a job goes through as it is processed.– Sample output of messages & results.



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Thank You!

MVS (Day 2). Copyright © 2005, Infosys Technologies Ltd 2 ER/CORP/CRS/OS01/003 Version No: 1.0...

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Transcript of MVS (Day 2). Copyright © 2005, Infosys Technologies Ltd 2 ER/CORP/CRS/OS01/003 Version No: 1.0...