Linux on IBM Z
Linux on zSeries is the preferred term for the Linux operating system running on IBM mainframes, especially zSeries servers. Other terms with the same meaning include Linux/390, Linux for zSeries, and z/Linux.
Brief history
Linux on zSeries originated as two separate efforts to port Linux to IBM's largest servers. The first effort, the "Bigfoot" project, was an independent distribution, now stagnant. IBM published a collection of patches and additions to the Linux 2.2.13 kernel on December 18, 1999, to start today's mainline Linux on zSeries. Formal product announcements quickly followed in 2000, including the Integrated Facility for Linux (IFL) engines.
Hardware
IBM zSeries servers can run mixed workloads, including numerous other operating systems. Both hardware and software co-evolved over decades to support these workloads, and Linux is no exception.
IFLs are mainframe processors dedicated to running Linux. Microcode restricts IFLs from running "traditional" workloads, such as z/OS. z/OS software vendors do not typically charge for IFL capacity, and IFL hardware is less expensive than general purpose engines (CPs). Consequently businesses and governments can expand their mainframe Linux installations without affecting most of their software license charges. Although Linux can technically operate on both CPs and IFLs, IFLs simply cost less.
Linux on zSeries is available in both 31-bit and 64-bit versions, with the 64-bit versions rapidly gaining popularity as zSeries mainframes become more prevalent. The Linux 2.6.x kernel added substantial support for mainframe hardware, although several vendor distributions "backported" to the Linux 2.4.x kernel. For example, Linux on zSeries supports ESCON, FICON, SCSI-attached storage devices, and zSeries cryptographic accelerators.
Advantages
As the most scalable platform for Linux computing, zSeries offers several major advantages. First, companies and governments can consolidate numerous small Linux and PC servers, reducing operating costs. File servers, Web servers, print servers, name/directory servers, and other "utility" servers are prime candidates for server consolidation. Second, Linux on zSeries can take advantage of most well-known mainframe reliability and security features, to support continuous business operations. Third, IBM's mainframe customers benefit, like other Linux users, from the vast and growing portfolio of useful software applications, including open source innovations.
zSeries servers start at "under $200,000" (zSeries Model 110, U.S. 2004 list price, excluding disk storage), so Linux on zSeries is not appropriate for small businesses with fewer than about 30 Linux servers. However, beyond that threshold Linux on zSeries can become cost advantageous.
z/OS still has numerous reliability and availability advantages over Linux, so most Linux on zSeries customers run both operating systems and will likely do so for many years (if not decades).
Environment
zSeries servers have multiple levels of virtualization, including LPARs (flexible partitions). Linux on zSeries can occupy whole partitions — even whole servers. However, most Linux on zSeries customers take advantage of z/VM. z/VM provides "virtual machines," with Linux running as a guest operating system under a hypervisor. z/VM dynamically manages processors, memory, and storage on behalf of Linux, carving up a large mainframe into hundreds or even thousands of concurrently operating Linux images.
Since the Linux kernel tends to assume that it "owns" the entire system, including all its memory, z/VM helps "tame" Linux. It is likely that, over time, the Linux kernel will more closely cooperate with z/VM, to better manage memory in order to further increase capacity.
Support
Like other platform versions of Linux, Linux on zSeries is governed by the GPL, an open source software license. Linux on zSeries source code is available from numerous sources on a free and equal basis, and architectural support is now part of the mainline kernel effort. IBM dedicates several of its programming staff to the community effort, but IBM is by no means the only participant.
Most Linux on zSeries customers, particularly those with business-critical production workloads, purchase a software support contract from commercial Linux vendors such as SuSE or RedHat. IBM Global Services also offers support contracts, including 24x7 coverage.
Most Linux software applications are readily available for Linux on zSeries, including popular enterprise software packages such as WebSphere, and DB2 and Oracle databases.
Market outlook
Linux itself is growing rapidly as of late 2004, and Linux on zSeries is no exception. As businesses and governments grow their Linux-based computing infrastructures, increasingly large zSeries servers make financial and business sense. IBM is aggressively and successfully promoting Linux on zSeries, and the company is well regarded within the Linux community as a defender of open source values, particularly in its battles against SCO.
SuSE currently has the most popular Linux on zSeries distribution, although RedHat is close behind. Debian is another notable Linux on zSeries distribution.
Developer resources
Linux software developers that certify for zSeries can appeal to large enterprises. There are few barriers to doing so.
IBM offers a no-charge 30-day Linux on zSeries "test drive", allowing Linux developers of any size access to a live Linux guest running under z/VM for compiling and testing software.
There are at least two software-based mainframe emulation packages that can execute zSeries software, including Linux on zSeries. FLEX-ES from Fundamental Software is the officially sanctioned (legally blessed) option. Hercules is also available but is not sanctioned for running licensed mainframe operating systems, such as z/OS.