Diagrams and descriptions outline three different levels of system configurations.

The first two examples, in Figure 1, show that mainframe refers more to a style of computing rather than to unique hardware. Two different systems are illustrated and neither uses mainframe hardware in the generally accepted sense of the term.

Figure 1. Very small mainframe configurations

The first system illustrated is an IBM® Multiprise® 3000 system (MP3000). which IBM recently withdrew from marketing. It was the smallest S/390® system produced in recent years. The MP3000 has one or two S/390 processors plus a SAP processor. It also has internal disk drives that can be configured to operate as normal IBM 3390 disk drives. A minimal internal tape drive is normally used for software installation. The MP3000 can have a substantial number of ESCON® or parallel channels for connection to traditional external I/O devices.

The MP3000 is completely compatible with S/390 mainframes, but lacks later zSeries® features. It can run early versions of z/OS® and all prior versions of the operating system. It is typically used with z/VM® or z/VSE™ operating systems.

The second system shown, the emulated zSeries system, has no mainframe hardware. It is based on a personal computer (running Linux® or UNIX®) and uses software to emulate z/OS. Special PCI channel adapters can be used to connect to selected mainframe I/O devices. The personal computer running the emulated z/OS can have substantial internal disks (typically in a RAID array) for emulating IBM 3390 disk drives.

Both of these systems lack some features found in "real" mainframes. Nevertheless, both are capable of doing quality work. Typical application software cannot distinguish these systems from real mainframes. In fact, these are considered mainframes because their operating systems, their middleware, their applications, and their style of usage are the same as for larger mainframes. The MP3000 can be configured with LPARs and might run both test and production systems. The emulated system does not provide LPARs, but can accomplish much the same thing by running multiple copies of the emulator software.

A key attraction of these systems is that they can be a "mainframe in a box." In many cases no external traditional I/O devices are needed. This greatly reduces the entry-level price for a mainframe system.

Figure 2 shows a modest mainframe system and shows the typical external elements needed. The particular system shown is an IBM z890 system with two recent external disk controllers, a number of tape drives, printers, LAN attachments, and consoles.

This configuration is somewhat idealized in that no older devices are involved. The systems outlined here might have a number of LPARs active, for example:

• A production z/OS system running interactive applications.
• A second production z/OS devoted to major batch applications. (These applications also could be run in the first LPAR, but some installations prefer a separate LPAR for management purposes.)
• A test z/OS version for testing new software releases, new applications, and so forth.
• One or several Linux partitions, perhaps running Web-related applications.

Figure 2. Medium mainframe configuration

The disk controllers in Figure 2 contain a large number of commodity drives running in multiple RAID configurations. The control unit transforms their interfaces to appear as standard IBM 3390 disk drives, which is the most common disk appearance for mainframes. These disk control units have multiple channel interfaces and can all operate in parallel.

Figure 3 shows a larger mainframe, although this is still a modest configuration when compared to a large mainframe installation. This example is typical in that both older and newer mainframes are present, along with channel switches allowing all systems to access most I/O devices. Likewise, new and older disk controllers (and devices) and tape controllers (and devices) are present. The total system is in a modest Parallel Sysplex® configuration.

Figure 3. Moderately large mainframe configuration

Briefly, the devices in Figure 3 include:

• An IBM 3745, which is a communications controller optimized for connection to remote terminals and controllers, and LANs. A 3745 appears as a control unit to the mainframe.
• IBM 3490E tape drives, which, though somewhat outdated, handle the most widely used mainframe-compatible tape cartridges.
• A sixth-generation mainframe design (G6).
• A newer z990 mainframe.
• An Enterprise Storage Server® (ESS).
• ESCON directors.
• OSA Express connections to several LANs.
• A coupling facility (CF), which is shown as a separate box, but it might be an LPAR in the mainframe.