Editing Mainframe computer (section)

==Characteristics==
[[File:IBM System Z9 (type 2094 inside).jpg|right|thumb|280px|Inside an [[IBM System z9]] mainframe that has an IBM [[ThinkPad]] integrated into the system as a [[Hardware Management Console]] (HMC). The HMC is used by the [[Computer operator|operator]] to control, e.g., the hardware, the [[PR/SM]] configuration. A secondary function is to serve as a low performance [[Computer terminal#System console|operator console]] via a proprietary interface. The HMC is not supported as a terminal, and remote access to it is limited to HTTP. Two other ThinkPads serve as [[IBM System z9#Support Element & HMC|Support Elements]] and backup HMCs]]

Modern mainframes can run multiple different instances of operating systems at the same time. This technique of [[virtual machine]]s allows applications to run as if they were on physically distinct computers. In this role, a single mainframe can replace higher-functioning hardware services available to conventional [[Server (computing)|servers]]. While mainframes pioneered this capability, virtualization is now available on most families of computer systems, though not always to the same degree or level of sophistication.<ref>{{cite web|url=http://www.computerworld.com/article/2551154/virtualization/emulation-or-virtualization-.html|title=Emulation or Virtualization?|work=Computerworld |date=June 22, 2009 |last1=Kay |first1=Russell }}</ref>

Mainframes can add or [[hot swap]] system capacity without disrupting system function, with specificity and granularity to a level of sophistication not usually available with most server solutions.{{citation needed|date=July 2011}} Modern mainframes, notably the [[IBM Z]] servers, offer two levels of [[Hardware virtualization|virtualization]]: logical partitions ([[logical partition|LPARs]], via the [[PR/SM]] facility) and virtual machines (via the [[z/VM]] operating system). Many mainframe customers run two machines: one in their primary data center and one in their [[IT disaster recovery|backup data center]]—fully active, partially active, or on standby—in case there is a catastrophe affecting the first building. Test, development, training, and production workload for applications and 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. In practice, many customers use multiple mainframes linked either by [[IBM Parallel Sysplex|Parallel Sysplex]] and shared [[Direct-access storage device|DASD]] (in IBM's case),{{citation needed|date=July 2011}} or with shared, geographically dispersed storage provided by EMC or Hitachi.

Mainframes are designed to handle very high volume input and output (I/O) and emphasize throughput computing. Since the late 1950s,<ref group=NB>E.g., the [[IBM 709]] had channels in 1958.</ref> mainframe designs have included subsidiary hardware<ref group=NB>Sometimes computers, sometimes more limited.</ref> (called [[System/360#Channels|''channels'']] or [[CDC 6600#Peripheral Processors (PPs)|''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 [[database]]s and files. [[Gigabyte]] to [[terabyte]]-size record files are not unusual.<ref>{{cite press release |url=http://www.wintercorp.com/PressReleases/ttp2005_pressrelease_091405.htm |title=Largest Commercial Database in Winter Corp. TopTen Survey Tops One Hundred Terabytes |access-date=May 16, 2008 |archive-url=https://web.archive.org/web/20080513110826/http://www.wintercorp.com/PressReleases/ttp2005_pressrelease_091405.htm |archive-date=May 13, 2008 |url-status=dead }}</ref> Compared to a typical PC, mainframes commonly have hundreds to thousands of times as much [[computer storage|data storage]] online,<ref>{{cite report|url=http://www.treasury.gov/tigta/auditreports/2006reports/200620056fr.html|title=Improvements in Mainframe Computer Storage Management Practices and Reporting Are Needed to Promote Effective and Efficient Utilization of Disk Resources|first=Michael R.|last=Phillips|date=May 10, 2006|quote=Between October 2001 and September 2005, the IRS' mainframe computer disk storage capacity increased from 79 terabytes to 168.5 terabytes.|archive-url=https://web.archive.org/web/20090119212407/http://www.treasury.gov/tigta/auditreports/2006reports/200620056fr.html|archive-date=January 19, 2009|url-status=dead}}</ref> and can access it reasonably quickly. Other server families also offload I/O processing and emphasize throughput computing.

Mainframe [[return on investment]] (ROI), like any other computing platform, is dependent on its ability to scale, support mixed workloads, reduce labor costs, deliver uninterrupted service for critical business applications, and several other risk-adjusted cost factors.

Mainframes also have execution integrity characteristics for [[fault tolerant]] computing. For example, z900, z990, System z9, and System z10 servers effectively execute result-oriented instructions twice, compare results, arbitrate between any differences (through instruction retry and failure isolation), then shift workloads "in flight" to functioning processors, including spares, without any impact to operating systems, applications, or users. This hardware-level feature, also found in HP's [[NonStop (server computers)|NonStop]] systems, is known as lock-stepping, because both processors take their "steps" (i.e. instructions) together. Not all applications absolutely need the assured integrity that these systems provide, but many do, such as financial transaction processing.{{Citation needed|date=July 2011}}