Chapter 5. Disk and Tape Storage Considerations

The VSE/SP and VSE/ESA systems and MVS and OS/390 systems have some conceptual similarities and data compatibilities for disk and tape files. This chapter will discuss the similarities and differences between the VSE and OS/390 environments in the following areas:

5.1, Access Method Similarities and Differences

5.2, Data Set Naming Considerations

5.3, Storage and Space Management

5.4, Tape Similarities and Differences

5.5, DASD Similarities and Differences

5.6, VSAM Differences

These topics will be discussed in order within this chapter.

5.1 Access Method Similarities and Differences

5.1.1 Access Methods

An access method is a set of user application programming interfaces (APIs), utility programs, other programming, and format standards which provide users with the ability to readily store and retrieve data within a computer system.

The VSE/ESA and OS/390 operating systems each support a number of access methods, with varying levels of compatibility. Further, details of the implementation are different between the operating systems, even if the function and the external format of the data are the same.

Access methods used for disk and tape storage in the VSE system include the following:

SAM Sequential Access Method -- used for disk and tape devices. Records are stored and/or retrieved in the order presented.

In OS/390, the most similar access methods are QSAM or SAM-E.

ISAM Indexed Sequential Access Method -- formerly used for disk devices, when records were to be maintained (logically) in ascending key sequence, but might need to be retrieved in arbitrary order (by key). Obsolete, replaced by VSAM in most VSE environments over twenty years ago.

In OS/390, may still be supported, but not recommended.

VSAM Virtual Storage Access Method -- used for disk devices. Records can be stored and/or retrieved in the order presented, or in key or address order.

In OS/390, DFP/VSAM is the most similar access method.

VSAM will be discussed in a separate sub-chapter below.

© Copyright IBM Corp. 1998

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IBM OS/390 manual Disk and Tape Storage Considerations, Access Method Similarities and Differences Access Methods

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.