OS/390 VSAM Cross-Region SHR(4)

VSE VSAM SHR(4 x) will refresh buffers from disk for every read I/O, and will also lock the record, CI, or CA as appropriate to protect the file and user data from corruption by possible concurrent update activity. SHR(4) data sets can have inserts or updates which may cause CI and CA splits, and secondary allocations can also safely be handled by VSE/VSAM.

OS/390 VSAM SHR(4 x) works very differently than VSE/VSAM. When SHR(4 x) is specified OS/390 VSAM takes only the following special actions:

OS/390 VSAM will refresh the buffers from DASD for random reads. OS/390 VSAM will write the CI to DASD following the CHECK for random writes. This allows some read and write integrity protection. However, OS/390 VSAM does not automatically enqueue on the record, CI, or CA as VSE/VSAM does. Hence another user may simultaneously update the same record or control interval. Also OS/390 VSAM does not refresh buffers during sequential processing.

When SHR(4 4) is specified, a change in the High-Used-RBA of any component (data or index) is not allowed. For a KSDS, this means:

1.no CA splits are allowed

2.the high-key CI cannot be extended

For all types of data sets, no extensions or new extent allocations are allowed. If a program causes any of the above conditions, it will receive a ²no space² error code.

These restrictions provide some protection since it is not possible for multiple users to cause concurrent CA splits or extend the data set. However, without user programming of Assembler ENQ/DEQ macros it is still possible to:

1.lose updates

2.read back-level records

3.cause data set failures during concurrent CI splits

Of course, even with user ENQ/DEQ programming, it is possible for errors to cause the same loss of data conditions.

Control Block Update Facility (CBUF) is used if SHR(3 3) or SHR(4 3) is used with JCL DISP=SHR. This removes the programming restrictions related to updating the High-Used-RBA, stated above. It does not assure full read or write integrity.

With SHR(4 3), buffers are refreshed for each direct request.

Note

To provide equivalent VSE/VSAM SHR(4 x) protection, when multiple users are updating the same data set from different address spaces, the OS/390 VSAM SHR(4) user should read the chapter entitled ²Sharing a VSAM Data Set² in the manual DFSMS/MVS Using Data Sets, SC26-4922.

In addition, partial to complete solutions for this functional difference between VSE and OS/390 are available from software vendors which provide functions similar to VSE/VSAM SHAREOPTION(4).

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IBM manual OS/390 Vsam Cross-Region SHR4
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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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