What is a Mainframe?

Today, computer manufacturers don’t always use the term mainframe to refer to mainframe computers. Instead, most have taken to calling any commercial-use computer—large or small—a server, with the mainframe simply being the largest type of server in use today. IBM, for example, refers to its latest mainframe as the IBM System z9™ server. We use the term mainframe in this text (blog) to mean computers that can support thousands of applications and input/output devices to simultaneously serve thousands of users.

Servers are proliferating. A business might have a large server collection that includes transaction servers, database servers, e-mail servers and Web servers.
Very large collections of servers are sometimes called server farms (in fact, some data centers cover areas measured in acres). The hardware required to perform a server function can range from little more than a cluster of rack-mounted personal computers to the most powerful mainframes manufactured today.

A mainframe is the central data repository, or hub, in a corporation’s data processing center, linked to users through less powerful devices such as workstations or terminals. The presence of a mainframe often implies a centralized form of computing, as opposed to a distributed form of computing.

Centralizing the data in a single mainframe repository saves customers from having to manage updates to more than one copy of their business data, which increases the likelihood that the data is current.

Starting around 1990, mainframe processors and most of their I/O devices became physically smaller, while their functionality and capacity continued to grow. Mainframe
systems today are much smaller than earlier systems—about the size of a large refrigerator.

In some cases, it is now possible to run a mainframe operating system on a PC that emulates a mainframe. Such emulators are useful for developing and testing business applications before moving them to a mainframe production system.

Clearly, the term mainframe has expanded beyond merely describing the physical characteristics of a system. Instead, the word typically applies to some combination of the following attributes:
- Compatibility with mainframe operating systems, applications, and data.
- Centralized control of resources.
- Hardware and operating systems that can share access to disk drives with other systems, with automatic locking and protection against destructive simultaneous use of disk data.
- A style of operation, often involving dedicated operations staff who use detailed operations procedure books and highly organized procedures for backups, recovery, training, and disaster recovery at an alternative location.
- Hardware and operating systems that routinely work with hundreds or thousands of simultaneous I/O operations.
- Clustering technologies that allow the customer to operate multiple copies of the operating system as a single system. This configuration, known as Parallel Sysplex, is analogous in concept to a UNIX cluster, but allows systems to be added or removed as needed, while applications continue to run. This flexibility allows mainframe customers to introduce new applications, or discontinue the use of existing applications, in response to changes in business activity.
- Additional data and resource sharing capabilities. In a Parallel Sysplex, for example, it is possible for users across multiple systems to access the same databases concurrently, with database access controlled at the record level.

As the performance and cost of such hardware resources as central processing unit (CPU) power and external storage media improve, and the number and types of devices that can be attached to the CPU) increase, the operating system software can more fully take advantage of the improved hardware. Also, continuing improvements in software functionality help drive the development of each new generation of hardware systems.

(Dari Introduction to the New Mainframe: z/OS Basics)