Basic of Mainframe Mainframe computer Mainframe is a very large and expensive computer capable of supporting hundreds, or even thousands, of users simultaneously. In the hierarchy that starts with a simple microprocessor at the bottom and moves to supercomputers at the top, mainframes are just below supercomputers. In some ways, mainframes are more powerful than supercomputers because they support more simultaneous programs. But supercomputers can execute a single program faster than a mainframe. The distinction between small mainframes and minicomputers is vague, depending really on how the manufacturer wants to market its machines. Modern mainframe computers have abilities not so much defined by their single task computational speed (usually defined as MIPS ² Millions of Instructions Per Second) as by their redundant internal engineering and resulting high reliability and security, extensive input- output facilities, strict backward compatibility with older software, and high utilization rates to support massive throughput. These machines often run for years without interruption, with repairs and hardware upgrades taking place during normal operation. Software upgrades are only non-disruptive when Parallel Sysplex is in place, with true workload sharing, so one system can take over another's application, while it is being refreshed. More recently, there are several IBM mainframe installations that have delivered over a decade of continuous business service as of 2007, with hardware upgrades not interrupting service. Mainframes are defined by high availability, one of the main reasons for their longevity, because they are typically used in applications where downtime would be costly or catastrophic. The term Reliability, Availability and Serviceability (RAS) is a defining characteristic of mainframe computers. Proper planning (and implementation) is required to exploit these features. In the 1960s, most mainframes had no interactive interface. They accepted sets of punch cards, paper tape, and/or magnetic tape and operated solely in batch mode to support back office functions, such as customer billing. Teletype devices were also common, at least for system operators. By the early 1970s, many mainframes acquired interactive user interfaces and operated as timesharing computers, supporting hundreds or thousands of users simultaneously along with batch processing. Users gained access through specialized terminals or, later, from personal computers equipped with terminal emulation software. Many mainframes supported graphical terminals (and terminal emulation) by the 1980s. Nowadays most mainframes have partially or entirely phased out classic terminal access for end-users in favor of Web user interfaces. Developers and operational staff typically continue to use terminals or terminal emulators. Historically, mainframes acquired their name in part because of their substantial size, and because of requirements for specialized heating, ventilation, and (air conditioning HVAC), and electrical power. Those requirements ended by the mid-1990s with CMOS mainframe designs replacing the older bipolar technology. In a major reversal, IBM now touts its newer mainframes' www.pankajkumarjha.com ability to reduce data center energy costs for power and cooling, and the reduced physical space requirements compared to server farms. Characteristics of mainframes Nearly all mainframes have the ability to run (or host) multiple operating systems, and thereby operate not as a single computer but as a number of virtual machines. In this role, a single mainframe can replace dozens or even hundreds of smaller servers. While mainframes pioneered this capability, virtualization is now available on most families of computer systems, though not to the same degree or level of sophistication. The distinction between supercomputers and mainframes is not a hard and fast one, but supercomputers generally focus on problems which are limited by calculation speed while mainframes focus on problems which are limited by input/output and reliability ("throughput computing") and on solving multiple business problems concurrently (mixed workload). The mainframe is the backbone of many industries that are the lifeblood of the global economy. More mainframe processing power is being shipped now than has ever been shipped. Businesses that require unparalleled security, availability, and reliability for their applications depend on mainframe. Mainframes were designed initially for high-volume business transactions and, for more than 40 years, have been continually enhanced to meet the challenges of business data processing. No computing platform can handle a diversity of workloads better than a mainframe. But aren't "insert-your-favorite-alternative-platform" computers cheaper/faster/easier to operate? The answer is: It all depends. A student who is composing his term paper does not have the same information needs as a bank that needs to handle millions of transactions each day, especially because the bank also needs to be able to pass security and accounting audits to verify that each account has the correct balance. Mainframes aren't for every computing task. Businesses opt for mainframes and mainframe operating systems when they have large volumes of data, large transaction volumes, large data transfer requirements, a need for an extremely reliable system, or many differing types of workloads that would operate best if they were located on the same computer. Mainframes excel in these types of environments. Mainframe Computers Mainframes are computers used mainly by large organizations for critical applications, typically bulk data processing such as census, industry and consumer statistics, ERP, and financial transaction processing. The term probably had originated from the early mainframes, as they were housed in enormous, room-sized metal boxes or frames. Later the term was used to distinguish high-end commercial www.pankajkumarjha.com machines from less powerful units. Today in practice, the term usually refers to computers compatible with the IBM System/360 line, first introduced in 1965. (IBM System z10 is the latest incarnation.) Otherwise, large systems that are not based on the System/360 are referred to as either "servers" or "supercomputers". However, "server", "supercomputer" and "mainframe" are not synonymous (see client-server). Some non-System/360-compatible systems derived from or compatible with older (pre-Web) server technology may also be considered mainframes. These include the Burroughs large systems, the UNIVAC 1100/2200 series systems, and the pre-System/360 IBM 700/7000 series. Most large-scale computer system architectures were firmly established in the 1960s and most large computers were based on architecture established during that era up until the advent of Web servers in the 1990s. There were several minicomputer operating systems and architectures that arose in the 1970s and 1980s, but minicomputers are generally not considered mainframes.Many defining characteristics of "mainframe" were established in the 1960s, but those characteristics continue to expand and evolve to the present day. Characteristics of Mainframe Computers Nearly all mainframes have the ability to run (or host) multiple operating systems, and thereby operate not as a single computer but as a number of virtual machines. In this role, a single mainframe can replace dozens or even hundreds of smaller servers. While mainframes pioneered this capability, virtualization is now available on most families of computer systems, though not to the same degree or level of sophistication. Mainframes can add or hot swap system capacity non disruptively and granularly, again to a level of sophistication not found on most servers. Modern mainframes, notably the IBM zSeries, System z9 and System z10 servers, offer three levels of virtualization: logical partitions (LPARs, via the PR/SM facility), virtual machines (via the z/VM operating system), and through its operating systems (notably z/OS with its key-protected address spaces and sophisticated goal- oriented workload scheduling, but also Linux, OpenSolaris and Java). This virtualization is so thorough, so well established, and so reliable that most IBM mainframe customers run no more than two machines one in their primary data center, and one in their backup data center²fully active, partially active, or on standby²in case there is a catastrophe affecting the first building. All test, development, training, and production workload for all applications and all databases can run on a single machine, except for extremely large demands where the capacity of one machine might be limiting. Such a two-mainframe installation can support continuous business service, avoiding both planned and unplanned outages. Mainframes are designed to handle very high volume input and output (I/O) and emphasize throughput computing. Since the mid-1960s, mainframe designs have included several subsidiary computers (called channels or peripheral processors) which manage the I/O devices, leaving the CPU free to deal only with high-speed memory. It is common in mainframe shops to deal with massive databases and files. Giga-record or tera-record files are not unusual. Compared to a www.pankajkumarjha.com typical PC, mainframes commonly have hundreds to thousands of times as much data storage online, and can access it much faster. While some other server families also offload certain I/O processing and emphasize throughput computing, they do not do so to