5.6.6.3 Cross-System and DASD Sharing

You are in a cross-system sharing environment whenever you allow more than one copy of any operating system to access the same DASD volume concurrently. This includes multiple OS/390 guests running under VM or PR/SM.

You must not attempt to update via DASD sharing between VSE and OS/390

systems. The methods of DASD sharing protection are totally incompatible and you risk contamination or loss of:

data data sets catalogs VTOCs

Should you be planning to share ICF catalogs or VSAM data sets between two or more OS/390 systems, you should read the chapter entitled ²Sharing a VSAM Data Set² in the Using Data Sets manual. How catalogs are shared is documented in Managing Catalogs. The volume the catalogs are allocated on must be defined as SHARED to all images and the shareoptions for the catalog must be 3,4.

OS/390 Global Resource Serialization (GRS) and the new Record Level Sharing feature (part of SYSPLEX support) can provide additional cross-system and DASD sharing capabilities. For more information on GRS, refer to OS/390 V1R1.0 MVS Planning - Global Resource Serialization, GC28-1818, or OS/390 V1R3.0 MVS Planning - Global Resource Serialization, GC28-1759.

OS/390 Definitions for DASD Sharing Support

In order to provide any protection in a DASD sharing environment you must let OS/390 know that the device may be shared. This is done by specifying the parameter ²SHARED² in the Hardware Configuration Definition data set. Without this parameter, OS/390 will provide no DASD sharing protection.

OS/390 VSAM Cross-System Shareoptions

Unlike VSE/VSAM, the cross-region shareoption has no meaning for

cross-system sharing with OS/390 VSAM unless OS/390 GRS is used to control cross-region sharing of data sets. What is important is the second field of the SHR parameter. This field can only be 3 or 4. In other words, only SHR(x 3) or SHR(x 4) is valid for an AMS DEFINE or ALTER.

Record Level Sharing (RLS), available to users of OS/390 systems with Parallel Sysplex capabilities (Coupling Facility, integrated or stand-alone), together with DFSMS support included in DFSMS 1.3 or later provides a much higher level of sharing with protection for OS/390 users. It is an alternative to using shareoptions for accessing VSAM data sets.

RLS can be used in batch environments but there are restrictions. RLS is designed primarily for VSAM data sets used by CICS applications. With VSAM RLS, multiple CICS systems can directly access a shared VSAM data set, eliminating the need for function shipping between AORs and file owning regions (FORs).

Information on Record Level Sharing can be found in several manuals:

Chapter 5. Disk and Tape Storage Considerations 129

Page 153
Image 153
IBM Cross-System and Dasd Sharing, OS/390 Definitions for Dasd Sharing Support, OS/390 Vsam Cross-System Shareoptions

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.