5.6.6.1 Cross-Region Sharing - Single CPU Environment

Whenever a VSAM data set (ACB) is opened by more than one control block structure concurrently, data integrity must be considered. OS/390 VSAM offers two levels of protection and/or sharing within a single CPU.

1.OS/390 will prevent concurrent update/update or update/read access to a VSAM data set if DISP=OLD is coded on the JCL DD statement. If DISP=OLD is specified, the shareoptions will be treated as (1 3). This can potentially provide performance improvements for load jobs.

2.VSAM will monitor access via the data set shareoptions if DISP=SHR has been specified on the DD statement.

The purpose of the VSAM shareoptions is to permit the user to specify the required level of integrity of the data set and prevent possible loss of records, updates, or even total loss of access to the data set. There are actually two types of integrity that can be of concern and the shareoptions vary in the type of integrity provided:

∙

∙

Write integrity - Assurance that, if an update or add is done, it will not be lost and the data set will not be destroyed.

Read integrity - Assurance that the record read is current (that is, no other user has since updated it).

Now let us review the shareoptions with respect to the integrity provided.

Integrity

Read Write

SHR(1 x) YES YES

SHR(2 x) NO YES

SHR(3 x) NO NO

SHR(4 x) NO* NO**

Figure 10. OS/390 VSAM Integrity Provided by Cross-Region Shareoptions

*VSE/VSAM will refresh buffers if a data set is open for output. VSE/VSAM also automatically enqueues on CIs and CAs to ensure integrity in the event of concurrent requests. OS/390 buffer refresh is done only for random (direct) reads. OS/390 VSAM does not automatically enqueue on records as VSE/VSAM does.

**VSE/VSAM SHR(4 x) guarantees the write integrity of a VSAM data set. OS/390 VSAM SHR(4 x) does not guarantee the write integrity of a VSAM data set.

With SHR(1 x) a user can not open a file for input to read if another user has opened it for output, so both read and write integrity are assured.

With SHR(2 x) read integrity is not assured because a program (user) may be accessing a data record from a buffer while another user is updating it on disk. Write integrity is assured since there can only be one update at a time.

With SHR(3 x) VSAM does not prevent any user′s open and does not monitor their access. Yes, you can destroy the file!

126VSE to OS/390 Migration Workbook

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IBM manual Cross-Region Sharing Single CPU Environment, OS/390 Vsam Integrity Provided by Cross-Region Shareoptions
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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.
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