IBM OS/390 manual Mergers/Acquisitions, Capacity Constraints, Virtual Storage

1.2.2 Mergers/Acquisitions

As with corporate consolidations, mergers and acquisitions present an equal number of challenges when having to incorporate the I/S organizations of the companies involved. A challenge that clearly presents itself is when the organizations involved run different host based operating systems (such as OS/390 and VSE/ESA). In cases where it has been decided to merge the I/S organizations rather than run as autonomous entities, the issue of which operating system should become the single production operating system arises. It is often decided that because of its robust/enhanced functionality the operating system be OS/390. This, then, requires that the VSE subsystems and applications be converted to OS/390.

1.2.3 Capacity Constraints

Users running DOS/VSE and/or VSE/SP encountered system capacity constraints due to the architectural design limits imposed by VSE. The need for additional system capacity and resources due to things such as application and end user growth found many VSE users coming up against these constraints. OS/390 provided the much needed relief for users who found themselves in this situation. Fortunately, with the introduction of VSE/ESA V1 many of these constraints were removed.

VSE users now find that many of the reasons, due to architectural limits, that forced a conversion to OS/390 actually no longer exist. The following sections describe some of these constraints in greater detail.

1.2.3.1 Virtual Storage

VSE/SP provided 24-bit addressing which supported 16 megabytes of virtual storage. Users with the requirement for a large CICS partition, for example, were forced to go to multiple CICS partitions when putting up a single large CICS partition was not possible. This sometimes caused additional problems as it was often difficult to split a single CICS application into multiple CICS partitions. However, where possible, users chose to implement multiple CICS regions using the CICS Multiple Region Option (MRO). Still, with the addition of multiple CICS regions (MROs), comes the added expense of managing the MROs. And, as the MROs numbers increase, you need system management tools, such as CICSPlex System Manager for MVS/ESA (CICSPlex SM) to ease the system management burden caused by multiple CICS systems.

MVS, or OS/390, provided users with virtual storage constraint relief through 31-bit addressing capabilities. However, some users found relief with virtual address extensions (VAE) in VSE/SP V3. VSE/ESA V1 introduced 31-bit addressing support. This now gives VSE users the ability to address up to 2GB of virtual storage. Hence, it is now possible for VSE users with large CICS partition requirements to have this requirement satisfied by VSE.

Chapter 1. Why Customers Migrate 5