26.6 Shared DASD vs. Cloned DASD

The issue of whether to share DASD volumes and data sets between systems is decided on the basis of DASD space availability, need for multiple versions of a file, and the ability to manage updates between the two systems.

26.6.1 Shared DASD between OS/390 Test Systems (vs. Cloned DASD)

The decision to share data sets and volumes or to make copies of them for each OS/390 system should be thought out carefully. Many OS/390 data sets and some volumes can be shared between multiple systems as long as updates are serialized and good change control procedures are followed. The recommended approach is to put multiple OS/390 systems in the same sysplex and use GRS to guarantee serialization of these resources. The alternative is to ²clone² or make copies of the volumes or data sets, but this obviously takes more DASD space.

Referring to Table 45 on page 403, there are some volumes that can be shared between active OS/390 systems, and others that should never be shared:

System Libraries Separate SYSRES volumes should be maintained for each logical OS/390 system.

Distribution Libraries Share, but only update from the maintenance system.

CatalogsThe master catalog should be fairly static, contain only the necessary entries, with the rest in user catalogs. The master catalog can be shared but highly controlled as to who can update it. User catalogs can be shared between systems, use GRS or manual procedures to serialize updates.

Paging Data Sets Use by only one system at a time (re-use by different test system as long as they are not both active at once.)

Spool & Checkpoint JES2 can share the spool and checkpoint between multiple members if you are comfortable with

multi-access spool. Otherwise, they should be dedicated to each OS/390 system. The backup OS/390 system should have its own spool and checkpoint.

Softcopy Library Share these between all OS/390 systems.

DFSMShsm ML1 Share if using HSM on both systems.

Storage/WorkShare or isolate depending on how much space you want to reserve for the ²production² system.

User LibrariesYou can share the user program and data libraries, but you must keep track of separate version levels with different data set names or different user catalogs and volumes.

A lot of these decisions whether to share volumes, data sets and work space depends on how much you want to isolate the systems and manage change control.

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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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