IBM OS/390 manual Our Recommendation

central storage as if it were running natively on the processor. While storage is dedicated to a particular virtual machine, it is unavailable to other virtual machines, and also VM/ESA itself.

VM/ESA however, provides much more than simply hypervisor, and virtualization functions. It also provides a full function, interactive, virtual machine operating system, CMS. CMS provides an unmatched interactive environment for program execution, and program development. In addition, VM/ESA provides communication methods that allow virtual machines to communicate with one another, as well as a shared spool for use by virtual machines. These capabilities go far beyond the hypervisor and hardware management functions provided by PR/SM, setting VM/ESA apart among operating systems.

Since the migration from VSE to OS/390 will involve the creation of many operating system images to support various facets of the migration process, speed and flexibility in defining these images is essential. In addition, for these operating system images to be useful, they cannot operate as islands, but rather must be interconnected in as many ways as possible. For example, the images may need to share DASD (even though this is very difficult for VSE and OS/390 to accomplish), may need to have VTAM connections to support NJE, file transfer, and logon sessions, and may need shared access to printing or tape resources. Spending a lot of time on these details diminishes the focus that can be applied to the task at hand, namely migrating from VSE to OS/390. Therefore, it is essential to be able to quickly and effectively bind images together without extensive disruption to the entire operating environment.

26.3.3 Our Recommendation

With these characteristics of the migration in mind, and based upon the very brief introduction to the two partitioning environments, it is the recommendation of this document that customers use software partitioning with VM/ESA whenever possible. VM/ESA will provide the partitioning flexibility, and speed of implementation necessary to support the migration process. In addition, CMS and other products such as RSCS, TCP/IP, and VTAM that are available to run natively on VM/ESA can further enhance the migration process beyond what would be possible in a PR/SM only environment.

Many VSE customers today have VM/ESA installed, and are using it as a production hypervisor, or a hypervisor for test virtual machines. For these customers, this recommendation is business as usual. For VSE customers who are not currently using VM/ESA, this recommendation may seem to pose more of a problem. However, before rejecting this idea out of hand as too costly or difficult to implement, consider that recent pricing actions make ownership of VM/ESA along with another S/390 operating system very attractive and affordable. In addition, improvements in VM/ESA installation automation make it very easy to install, typically taking no more than a couple of hours. Factor in also the potential savings associated with not having to buy some additional hardware to support more LPARs (such as channels, control units for consoles and so on), and this suggestion becomes very attractive. In addition, consider that the introduction of VM/ESA can be useful to your enterprise far beyond the migration process. VM/ESA can continue to be used to provide a test environment for OS/390 guests used in the maintenance and test process. As your enterprise grows, and you begin to explore functions such as parallel sysplex in the OS/390 environment, VM/ESA′s virtual coupling facility support can enable you to define and run a sysplex under a single VM image without any real coupling facilities being defined, or coupling links being purchased and

424VSE to OS/390 Migration Workbook