IBM OS/390 manual Logical Partitioning

evaluate each of the alternatives mentioned, but that would be beyond the scope of this book, and not necessarily relevant to the task at hand.

Since this document is directed toward migration of VSE systems to OS/390 primarily on CMOS processors, we will not consider the choices of physically partitioning one or more processors, since this option is not available on CMOS processors. In addition, we will not consider the choice of using separate hardware platforms for each system image since it is very often not a very practical alternative. Although, with the P/390 platform it is a much more attractive possibility than it ever used to be. Rather, we will limit our consideration to the choices of logical partitioning, software partitioning, or a combination of both.

An excellent comparison of the relative merits of logical partitioning and software partitioning with VM/ESA is contained in the IBM Redbook ES/9000 Multi-Image Processing from a VM/ESA Perspective Volume 1, GG24-3920. The reader is directed to this reference for a comprehensive comparison of LPAR and VM/ESA. The comparison provided is neutral in the sense that it is not considering any particular task other than running and supporting guest operating systems. In other words, the added consideration of which solution best supports a migration environment is not included. Thus we will attempt to add to that discussion from the perspective of migrating VSE systems to OS/390. It should be noted that this Redbook was written in late 1993, and as such is out of date in terms of hardware platforms discussed. However, the bulk of comparison items, and conclusions drawn are still very relevant to today′ s hardware platforms. Details such as the number of partitions supported on a particular hardware platform have changed, however, the basic notion that a finite number is supported through logical partitioning, and a substantially larger number of images is supported through software partitioning still holds true.

Before presenting a recommendation for the migration environment, it would be good to review briefly the characteristics of logical and software partitioning. This review will serve as a basis for the recommendations that follow.

26.3.1 Logical Partitioning

Logical partitioning is achieved through the use of Processor Resource System Manager (PR/SM) licensed internal code (LIC). Processors since the 3090 series have had this LIC available. However, in the 3090 days, it was most commonly a priced feature added on to the processor. With the general availability of the ES/9000 series processors, PR/SM has become a standard feature on S/390 processors. Multiple system images are supported by defining logical partitions, and assigning resources (CPU, Central Storage, Peripherals) to the partitions. This is accomplished through specifications made in the IOCP, and commands entered through the hardware system management console.

It is possible to define more partitions than the available physical resources can support, however, the number of partitions that can be active at any time is limited to what the physical resources can support. For example, if your Central Electronic Complex (CEC) has 1GB of central storage available, and you have defined five logical partitions each specifying 500MB of central storage, you are limited to activating two of the partitions at any one time. The remaining three partitions are defined, but cannot be activated until one of the currently activated partitions is deactivated.

422VSE to OS/390 Migration Workbook