Recent advances in the PR/SM LIC have solved some of the real storage management difficulties encountered in the past concerning assigning contiguous storage chunks to LPARs. In addition, ESCON Multi Image Facility (EMIF) channels have reduced the number of physical channels that must be installed to support multiple partitions by allowing for the sharing of physical channels between partitions. However, notwithstanding these recent advancements, the definition of partitions still requires the dedication of real hardware resource. As a consequence, logically partitioning a CEC will typically involve the purchase of more hardware than would be required to run the CEC in single image mode.

26.3.2 Software Partitioning

Software partitioning is accomplished through the installation and use of the VM/ESA operating system. VM/ESA manages the real hardware resources of a CEC, and makes it available to software defined virtual machines. The user of the VM/ESA operating system is provided with a virtual machine at logon time, and accesses all real hardware resources through that virtual machine. VM/ESA provides each virtual machine with storage, CPU resources, and peripheral devices. Often these resources are virtual (hence the name of the operating system). This means that peripheral devices, for example, are software constructs that emulate the real peripheral devices, and provide access to real underlying devices without dedicating that device to a particular virtual machine. The VM/ESA operating system manages the real devices such that the integrity of the devices is maintained, and individual virtual machines are prevented from accessing or destroying resources being used by another virtual machine.

Virtual machines are defined in a flat file known as the VM Directory. Each virtual machine is represented by a stanza within this file. A virtual machine definition consists of a name for the virtual machine (user ID) along with statements that define how many virtual CPUs are allocated, how much virtual storage is allocated, what virtual devices are allocated and their characteristics, along with privileges given to the virtual machine. Since the resources of a virtual machine are not real, the number of virtual machines that can be defined and active at any one time is not limited to the physical resources available. Using the example presented above for logical partitioning, it would be possible to log on all five of the virtual machines defined (each with 500MB of storage defined), even though the real CEC only has 1GB of central storage available. VM/ESA manages the use of real memory by paging real frames of central storage to slots on DASD based upon the use and reference patterns of the virtual machines.

Since software partitioning involves an operating system, some real hardware resources will need to be allocated to support this operating system. Thus the real resources of the CEC will now be distributed among three operating systems in our migration scenario instead of just two. It is possible however, to minimize the impact that VM/ESA has upon use of the real hardware resources. For example, instead of having VM/ESA create virtual DASD devices for a particular virtual machine, it is possible to dedicate certain real DASD devices to a virtual machine. This removes the simulation overhead incurred by having VM/ESA maintain a virtual device, and allows the virtual machine complete control over the particular DASD device. While the DASD device is dedicated to a particular virtual machine it is unavailable for use by any other virtual machine. It is also possible to dedicate central storage to a particular virtual machine. When this is done, it avoids the overhead of having VM/ESA perform paging operations, and allows the guest to occupy and manage a portion of real

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IBM OS/390 manual Software Partitioning

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.