Default Models, Noallocation Data Sets | IBM OS/390 specification

VSE SAM/VSAM data sets may also be converted to VSAM ESDS data sets. However, this is not recommended as it requires changes to the programs.

Default Models

Both VSE and OS/390 VSAM support the MODEL parameter of the DEFINE command. This allows the attributes of an existing file to be used during the define of a new file. VSE/VSAM also supports three types of model data sets through the AMS DEFINE NOALLOCATION command. OS/390 VSAM does not support the NOALLOCATION parameter. NOALLOCATION is usually used for:

∙Through reserved entry names, installation defaults may be specified for one or more VSAM file organizations. This is frequently used for SAM/VSAM files.

∙A specific model for a specific data set name. This is typically used to defer allocation until the file is opened. Space is not automatically freed when the file is closed.

NOALLOCATION Data Sets

Through the REUSE option, temporary VSAM data sets (dynamic files) may be defined. No space is allocated until the file is opened and space may be freed when the file is closed. OS/390 supports temporary VSAM data sets in the DFSMS environment.

DFSMS (Data Facility Storage Management Services) is highly recommended and its functions can greatly ease migration of several kinds of VSAM and non-VSAM data sets from VSE to OS/390.

JCL Implicit DEFINE

The default models allow VSE SAM/VSAM files to be defined via JCL, without the need for a specific AMS DEFINE. In OS/390, unless DFSMS is active, all VSAM data sets must be defined using AMS. DFSMS provides new OS/390 JCL keywords which allow some VSAM data sets to be implicitly defined in the JCL.

Reusable Data Sets

Both VSE and OS/390 VSAM support definition of reusable data sets. Usage of reusable data sets differs. VSE allows the REUSE option to be specified in AMS REPRO commands, in the ACB, or through the VSE JCL. OS/390 only supports the options specified in the ACB or through the AMS REPRO command options. OS/390 high level languages (for example, COBOL for OS/390 & VM) permit a reusable data set to be extended if it is opened with the EXTEND attribute, and for sequential access. For older language compilers, a method to extend a reusable data set under OS/390 would be to have the application write to a temporary file, then use an AMS REPRO with the REUSE option to copy it to the intended reusable data set. In neither case can this be controlled by JCL options in OS/390, as it can in VSE.

Two VSE examples are shown below:

To extend a reusable data set - / / DLBL .....,VSAM,DISP=OLD

Chapter 5. Disk and Tape Storage Considerations 123

OS/390 specifications

IBM OS/390, a versatile operating system, was a cornerstone in enterprise environments and played a pivotal role in mainframe computing. Released in the mid-1990s, OS/390 combined the strengths of IBM's MVS (Multiple Virtual Storage) with new features and enhancements, targeting scalability, reliability, and performance in demanding business applications.

One of the key features of OS/390 was its robust support for multiple users and processes. The system allowed thousands of concurrent users to access applications and data, ensuring high availability and minimizing downtime—a critical requirement for many large organizations. This scalability was supported through various enhancements in memory management and processor scheduling, enabling optimal resource allocation across diverse workloads.

OS/390 was known for its superior workload management capabilities. The Workload Manager (WLM) component allowed administrators to define service policies, specifying how system resources would be allocated according to the priority of tasks. This ensured that critical business processes received the necessary resources while less critical tasks were managed more flexibly.

Another significant characteristic of OS/390 was its commitment to security. The operating system provided comprehensive security features, including user authentication, data encryption, and auditing capabilities. This focus on security was vital for organizations handling sensitive data, ensuring compliance with regulations and safeguarding against unauthorized access.

OS/390 also supported advanced technologies that facilitated integration and development. The system included features like the IBM CICS (Customer Information Control System) for transaction processing and IMS (Information Management System) for database management. These technologies allowed organizations to build robust, high-performance applications tailored to specific business needs.

The ease of network integration was another strength of OS/390. With the advent of the Internet and global connectivity, OS/390 systems could easily interface with various network protocols, enabling businesses to operate in a connected world. This inclusion paved the way for many organizations to expand their capabilities and offer new services, driving digital transformation.

In conclusion, IBM OS/390 represented a significant advancement in mainframe technology, combining scalability, security, and robust workload management. Its rich feature set and support for critical enterprise applications solidified its role as a vital component of many organizations' IT infrastructures, ensuring they could meet their operational challenges head-on while supporting future growth. As technology continues to evolve, the legacy of OS/390 remains influential in the realm of computing.

IBM OS/390 manual Default Models, Noallocation Data Sets, JCL Implicit Define, Reusable Data Sets

Models: OS/390