flows. For example: You can have hundreds of files with the same name, for example ²WORK1². WORK1 can exist in different jobs. Most of the time the WORK1 files will be different from and unrelated to each other. You can however have JobA use a file named WORK1 and JobB running at the same time and also using a file called WORK1. JobA WORK1 gets passed to Job3. Job3 runs and uses WORK1 at a later time. WORK1 in JobB is local use only. The issue is to distinguish the differences between the WORK1 file in JobA and JobC. If these files become intermixed or used by the wrong Job it can be complicated to sort out. There is no data exchanged through those file references. Only analyzing the complete file/step cross-references with associated open modes and attributes allows identifying these situations. When migrating to OS/390, it is critical to identify those data flows as separate OS/390 files. In the worst case scenario, it would create execution or JCL errors. In the best case, it would create unwanted contentions, when concurrent jobs try to access the ²same² file: OS/390 will allow more job multi-threading than VSE, if contentions are not an issue.

Once the steps above are completed, ²Forward engineering² techniques are used to generate OS/390 JCL streams that match the application job streams while complying with new OS/390 standards and naming conventions. This is the easiest part of the VSE to OS/390 JCL conversion.

The conversion of VSE JCL to OS/390 may last three to five months and represent 40 to 50% of the total application conversion effort.

2.7.4 File Migration

File migration can only be as good as JCL conversion. This is because the most challenging parts of the file migration, identifying and classifying all files according to their life, and the tape to disk device migration, are for the most part a by-product of JCL conversion.

VSE files and databases can either be migrated ²in-place² or by copy. Both techniques can be combined in the overall VSE to OS/390 file migration strategy.

In-placemigration is by far the quickest, therefore the less disruptive (very short operations outage). Entire VSE data centers with extremely large application data pools have been migrated to OS/390 in an hour. But by altering the VSE production environment, it prevents instant return to VSE. It also complicates, limits or even prevents the implementation of DFSMS, at least at start: natural production cycles may be used to replace the sequential files migrated in place by new DFSMS-controlled versions.

File copy takes longer, but with appropriate configuration and planning, large VSE installations are routinely migrated in mass in only four to eight hours. If the VSE disk space is unaltered by the file migration, instant VSE fallback is possible. This technique facilitates full size DFSMS implementation from the very start of OS/390 operations.

Developing a file migration strategy and associated procedures (VSE and OS/390 file migration JCL streams) is not very difficult, technically speaking. Migrating VSE production files to OS/390 for conversion regression tests allows rehearsing and finalizing the procedures that will later on be used for the actual file migration and operations switchover.

Chapter 2. Sizing the Effort

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IBM OS/390 manual File Migration

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.