MVS

MVS (Multiple Virtual Storage) is an operating system from IBM that continues to run on many of IBM's mainframe and large server computers. MVS has been said to be the operating system that keeps the world going and the same could be said of its successor systems, OS/390 and z/OS. The payroll, accounts receivable, transaction processing, database management, and other programs critical to the world's largest businesses are usually run on an MVS or successor system. Although MVS has often been seen as a monolithic, centrally-controlled information system, IBM has in recent years repositioned it (and successor systems) as a "large server" in a network-oriented distributed environment, using a 3-tier application model.

The follow-on version of MVS, OS/390, no longer included the "MVS" in its name. Since MVS represents a certain epoch and culture in the history of computing and since many older MVS systems still operate, the term "MVS" will probably continue to be used for some time. Since OS/390 also comes with Unix user and programming interfaces built in, it can be used as both an MVS system and a UNIX system at the same time. A more recent evolution of MVS is z/OS, an operating system for IBM's zSeries mainframes. MVS systems run older applications developed using COBOL and, for transaction programs, CICS. Older application programs written in PL/I and FORTRAN are still running. Older applications use the Virtual Storage Access Method access method for file management and Virtual Telecommunications Access Method for telecommunication with users. The most common program environment today uses the C and C++ languages. DB2 is IBM's primary relational database management system (RDBMS). Java applications can be developed and run under OS/390's UNIX environment.

MVS is a generic name for specific products that included MVS/SP (MVS/System Product), MVS/XA (MVS/Extended Architecture), and MVS/ESA (MVS/Enterprise Systems Architecture). Historically, MVS evolved from OS/360, the operating system for the System/360, which was released in 1964. It later became the OS/370 and the System/370. OS/370 evolved into the OS/VS, OS/MFT, OS/MVT, OS/MVS, MVS/SP, MVS/XA, MVS/ESA, and finally OS/390 and then z/OS. Throughout this evolution, application programs written for any operating system have always been able to run in any of the later operating systems. (This is called forward compatibility.)

An MVS system is a set of basic products and a set of optional products. This allows a customer to choose the set of functions they need and exclude the rest. In practice, most customers probably use almost all of the functions. The main user interface in MVS systems is TSO (Time Sharing Option). The Interactive System Productivity Facility (ISPF) is a set of menus for compiling and managing programs and for configuring the system. The main work management system is either Job Entry Subsystem 2 or 3 (JES2 or JES3). Storage (DASD) management is performed by DFSMS (Distributed File Storage Management Subsystem). MVS is considerably more complex and requires much more education and experience to operate than smaller server and personal computer operating systems.

The Virtual Storage in MVS refers to the use of virtual memory in the operating system. Virtual storage or memory allows a program to have access to the maximum amount of memory in a system even though this memory is actually being shared among more than one application program. The operating system translates the program's virtual address into the real physical memory address where the data is actually located. The Multiple in MVS indicates that a separate virtual memory is maintained for each of multiple task partitions.

Other IBM operating systems for their larger computers include or have included: the Transaction Processing Facility (TPF), used in some major airline reservation systems, and VM, an operating system designed to serve many interactive users at the same time

VSAM

VSAM is a high-performance access method used in the MVS, OS/390 and VSE/ESA operating systems. It was initially released by IBM in 1973 and is part of the Base product.

VSAM provides a number of data set types or data organization schemes. They are:

• Key-sequenced data set (KSDS)
• Entry-sequenced data set (ESDS)
• Relative record data set (RRDS)
• Variable-length relative record data set (VRRDS)
• Linear data set (LDS)

Installations have been using VSAM data sets to hold more and more of their data to the point where many have reached the 4-gigabyte architectural limit for the size of VSAM data sets. Beginning with DFSMS V1.3, you can create and use VSAM KSDSs that can be much larger than the 4-gigabyte limit imposed on any VSAM data set defined before this release. DFSMS V1.5 allows non-KSDS file types (ESDS, RRDS, VRRDS and LDS) to exceed 4 gigabytes.

VSAM record-level sharing (RLS) was introduced to provide the value of the Parallel Sysplex to the existing applications. RLS itself does not provide transactional recovery. CICS provides a file access interface on top of VSAM. It is a CICS file control function that includes transactional recovery for VSAM files. This isolation and rollback capability enables VSAM data to be shared among CICS applications.

DB2

DB2 is an abbreviation for IBM Database 2 and was launched in June 1983 as a subsystem on MVS that allowed MVS users to build, access, and maintain relational databases using the well known Structured Query language (SQL).

Since then, DB2 has come a long way and provides facilities to exploit the latest hardware and software technologies, accommodating a majority of user requirements. The latest versions are available on almost all platforms, including Windows, HP-UX, Sun Solaris, Unix, AIX, NUMAQ, Linux, AS/400 and OS/390.

As the name suggests, DB2 "Universal Database" provides universal data types, universal integration, universal access from clients of all types, universal applicability (for all types of applications), universal scalability (across all types of platforms), universal reliability (for non-stop 24/7 processing) and universal manageability.

The ability to manage many concurrent users, very large databases, high transaction rates and deliver consistent rapid response is fundamental and delivered by DB2 through the wide range of platforms and the exploitation of platform-specific features. Beyond this, DB2 meets the requirements for high availability, low planned maintenance, wide connectivity, open standards and effective manageability.

Job Entry Subsystems:

Job Scheduler:

JES is an MVS component which keeps track of jobs that enter the system. The job entry subsystem also called as JES in short is used by MVS operating system. The JES is the component which presents jobs for MVS processing and sends the job’s spooled output back to the correct destination. A JOB is the execution of one or more related programs in sequence. Each program to be executed by a JOB is called a JOB STEP.

Types of JES:

There are two main types in this as given below:

HASP:

This is the acronym for Houston Automatic Spooling Program. It is otherwise called as JES2.

ASP:

This is the acronym for Asymmetric Multiprocessing system. It is otherwise called as JES3. This system is more suitable for shops with more than one processor.

It is vital that a user must know that each MVS system can use JES2 or JES3 but not both.

Job management of MVS:

The task of job management of MVS is taken care by two main resources:

JES and

Base control program of MVS.

Here the former job entry subsystem also called as JES in short takes up the task of managing jobs before and after running the program. The latter base control program takes the task of managing the job during processing.

General Phases in a Job:

The phases through which a job generally flows through starting from input stage till final stage are given below:

Input

Conversion

Processing

Output

Print

Purge

Input:

The HASP otherwise called as JES2 accepts jobs as input stream. The ability of JES2 is to accept multiple jobs at the same time. Before JES2 accept the jobs it is obvious that user has to submit the jobs. This process of submitting jobs can be done using programs and commands to JES2.We will see about commands in JES2 in detail in our coming sections. The jobs submitted is accepted as input stream by JES2 and a job identifier given to each JOB JCL statement with all jobs, JCL commands and statements being placed in spool data sets from which job is selected by JES2 for further processing.

Conversion:

In the process the jobs are processed and fine tuned for execution. That is to say in detail JES2 uses converter program which associates the JCL statements placed in JOB statement with JCL in procedure library. This merged JCL is then converted into a combined JCL as text by JES2. If there is no errors detected in JCL by JES2 the jobs are queued in spool date set for further processing and execution. If errors were detected by JES2 then appropriate messages are placed and the job is queued for processing and not for execution.

Processing:

In this the jobs that were placed in the previous phase in the jobs queue are taken by JES2 and sent to initiators and these are defined using JES2 initialization statements which we will see in detail in our JES2 commands section. The processing by initiators is based on various priorities of class assigned and the priority of queued jobs.

Output:

All the output produced in system by jobs also called as SYSOUT in short is controlled and monitored by JES2. In this phase the output activities like printing of datasets, output device handling, system messages to be outputted are all handled by JES2.The dataset which is to be outputted having same characteristics are grouped together by JES2 for printing.

Print:

In this phase the output dataset generated by earlier output phase are processed by JES2. Here the output is selected for processing by JES2 based on priority, output class mentioned on JCL. After JES2 process the output of job it places the job in purge queue for next phase to take over.