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Mainframes are powerful computers that can process extensive amounts of data and are highly reliable. Many commercial organizations and government systems use mainframes as servers rather than microcomputer-based servers. Most operating systems for mainframes are proprietary. However, thanks to Linux's open-source license and portable and modular design, Linux was ported to mainframe computers.
Before Linux was ported to mainframes, most mainframe operating systems were proprietary or closed-source. Some mainframe operating systems of the 1990s includes z/TPF, OS/390, AIX/370 (AIX for mainframes), and others. In December 1999, IBM submitted code to the Linux kernel that would help to later port Linux to mainframes. Not long after, Linux was successfully ported to mainframes (primarily, z/Architecture and IBM ESA/390 [a.k.a. s390] platforms).
Linux distros operating on a mainframe are commonly referred to as Linux/390, "Linux on zSeries", zLinux, or other terms. However, the Linux system is not truly running on the bare-metal system. Mainframes have excellent support for virtualization that is much better than microcomputers. Mainframes use their Processor Resource and System Manager (PR/SM) to run Linux on the system virtually. Because mainframes are very powerful (in terms of virtualization and I/O processing), mainframes can run several virtual instances of Linux. The System-Z hypervisor (called z/VM) can be used to execute additional instances of Linux.
In 2015, IBM releases a new set of z/Architecture mainframes that support and use Linux by default. This set of mainframes are called "LinuxONE Mainframes". LinuxONE is a project to make Linux work better on enterprise mainframes. IBM and Canonical Ltd. are working together to make Ubuntu support IBM's LinuxONE mainframes and zSeries mainframe hardware. When IBM first released its LinuxONE mainframes, IBM produced two types of LinuxONE systems - LinuxONE Emperor (IBM z13 based) and LinuxONE Rockhopper. LinuxONE Rockhopper is designed and intended for low-budget and entry-level projects and markets. In contrast, LinuxONE Emperor is more expensive and powerful.
In August 2015, The Linux Foundation announced the "Open Mainframe Project" (http://www.linuxfoundation.org/news-media/announcements/2015/08/linux-foundation-brings-together-industry-heavyweights-advance). On the started of the project, IBM contributed 250,000 lines of mainframe code to Linux.
Linux has some advantages over other mainframe operating systems. For instance, Linux is open-source freeware while other mainframe operating systems are not open-source or free to use. Linux is more secure than other mainframe systems. Linux also supports a wider variety of hardware than z/OS (a proprietary alternative to Linux for mainframes).
IFLs (Integrated Facility for Linux) are mainframe processors designed to run Linux natively on mainframes. IFLs are also less expensive than other IBM mainframe processors.
Non-mainframe Linux systems can cross-compile software to "z/Architecture binaries" which is used by zLinux. Traditional Linux software can be ported to the z/Architecture platform. However, the main porting issue is the endianness because zLinux uses big-endian. Programs written using scripting languages may not need to be ported if the script-interpreter has been ported to mainframes.
- LinuxONE - http://www.ibm.com/linuxone || http://www.ibm.com/systems/z/os/linux/linux-one.html
- Ubuntu for LinuxONE - http://ubunt.eu/systemz || https://insights.ubuntu.com/2015/08/17/ibm-and-canonical-plan-ubuntu-support-on-ibm-z-systems-mainframe/
- Porting Software to zLinux - https://github.com/linux-on-ibm-z/docs
- Over Five Types of Computers Explained and Compared - https://dcjtech.info/topic/over-five-types-of-computers-explained-and-compared/