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Linux on big iron

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When a small team of IBM programmers in Boeblingen, Germany completed a port of Linux on the IBM S/390 mainframe in December 1999, it ushered in a new era in the short history of Linux. Now Linux runs on everything from handheld PalmPilots to large mainframes.

At Linux World in early February 2000, IBM demonstrated Linux running on the S/390 by piping audio-streamed MP3s across the conference hall. IBM won’t say when–or even if–it will officially release the product, but the company is making the code available for download. And the notion of running Linux on an IBM mainframe has piqued the interest of at least some IBM customers. As of early March 2000, the code had been downloaded 600 times, according to IBM. And a Linux on S/390 e-mail list has more than 550 members.

So far, many of the mainframe users appear to be just kicking the tires. The Toronto Transit Commission, for example, is evaluating Linux for a number of uses. As part of the evaluation, Peter Webb, a technical support analyst at the Transit Commission, has been running Linux on the organization’s IBM mainframe, a Multiprise 2003-225, since January 2000. Webb hopes his test system will encourage the Transit Commission to consider the mainframe as a platform for Linux.

Money is one thing that may sway the Commission’s decision making. The Transit Commission may have to expand its raised floor computer room, says Webb, due in part to the increasing amount of space needed for Windows NT servers. Replacing those servers with copies of Linux running on the Transit Commission’s mainframe would decrease the amount of floor space required. “Since we can host multiple Linux servers within the mainframe box, we could avoid expensive computer room renovations,” Webb says.

Webb expects the Linux mainframe could be used for Web serving, e-commerce, databases, and file or print serving. “Basically, it is a good platform for anything where you need a really reliable server,” he says.

Other IBM mainframe users, such as the Oklahoma Department of Corrections in Oklahoma City, Okla., have firmer plans for Linux on the mainframe. Stephen Frazier, manager of technical services, expects to have a Linux mainframe running soon as a replacement for some of the corrections department’s servers. “We currently have about 40 Windows NT servers. If we were to replace them with Linux on our mainframe, we would have only one machine to support instead of 40. This would save us a lot of time and problems,” he says.

Test Plan Charlie

There are three ways to run Linux on an S/390: native, in a logical partitioning environment (LPAR), or under a virtual machine/enterprise systems architecture (VM/ESA). Running Linux natively means it is the only operating system and it controls the whole machine. When run in LPAR, the system resides on its own separate piece of the physical hardware, so that the machine can actually run several different operating systems on the same hardware. When running in a VM/ESA mode, a single underlying operating system controls the hardware but hosts one or more other operating systems, such as Linux, in virtual partitions.

VM/ESA is IBM’s operating system that allows you to simulate a complete computer environment in software. Multiple copies of Linux can run side-by-side on a mainframe under VM/ESA, each one operating as if it was on its own box. So far, most Linux mainframe users seem to favor running it under VM/ESA.

Just how many virtual copies of Linux can you run on an S/390? David Boyes, a Virginia-based engineer, has gained some notoriety in the Linux community for his “Test Plan Charlie” effort in which he ran 41,400 copies of Linux on the same S/390 CPU. “Each virtual machine was a complete multiuser network-accessible Linux system configured for a specific task,” he says. “Some generated Web traffic, some handled network connectivity, others were applications servers, others were shared file servers.” The system ran smoothly, although it did slow as more virtual systems were added. It did not crash, however, Boyes says, even when VM ran out of resources to allocate.

This sort of unlimited scalability, either on multiple virtual machines or on one copy of Linux, seems to be one of the things attracting users to Linux on the mainframe. “What if you could have a system that could run the same software as PC Linux, but could scale to almost any size you needed?” asked a systems programmer at a major medical center in the Midwest, who wished not to be identified by name because the Linux mainframe project at his company is in the preliminary stages. “Instead of 10 or 50 or 100 users, what if you could support thousands in the same Linux image?”

Another draw, says the Toronto Transit Commission’s Webb, is ease of use. Adding another Linux server to the mainframe under VM/ESA takes about 30 minutes, he says, and circumvents the need to buy new servers, run cables between machines, or even find a place to plug in a power cord, since the server operates in a virtual environment within the mainframe. And, Webb says, “connections between Linux servers inside a mainframe can be very fast, much faster than our existing Ethernet LAN.”

For his part, Boyes believes that one of the largest advantages of Linux/390 lies in the underlying reliability and high availability of the S/390 hardware platform. What’s more, he points out, “You don’t need a multimillion dollar network monitoring and system management solution–VM includes those tools as part of the base OS, and there are dozens of excellent monitoring solutions freely available.”

A time and a place for Linux S/390

Computing advances do not come without challenges, however. The lack of software support will be an issue for some time and will depend solely on the skills of the systems programmers, according to Bill Stephens, a VM systems engineer at a large mainframe datacenter in Philadelphia. Drivers will need to be written for several types of devices.

The Linux for S/390 code is available for download at IBM’s Linux for S/390 page.

To subscribe to the LINUX-VM e-mail discussion group, send an e-mail to //

As with any new system, Linux on the S/390 will face some of its biggest hurdles in perception. Marist College in Poughkeepsie, N.Y., has been at the forefront of the Linux S/390 efforts and is the first non-IBM site to run Linux S/390 from IBM. The site launched in December 1999. “For historical mainframe shops, they need to understand that Linux is a viable option for them, and for traditional Linux people, that Linux on 390 can bring things to the discussion,” says A. Harry Williams, director of technology and systems at the college. He emphasized that Linux on S/390 is the real thing. “It isn’t Linux-like or even Linux-lite. It is Linux,” he says.

But Linux on a mainframe is not the right tool for every job, says Scott Courtney, a senior technical analyst at a large manufacturing company and author of a series of articles on the Linux S/390. An S/390 running a light load will not run as quickly as a fast PC server under a light load, according to Courtney. The difference between the two systems will not be apparent until the load is much larger.

“The PC will begin to degrade and will typically reach a point where it avalanches down in performance as its load limit is exceeded. The mainframe starts out at a lower performance level, from the standpoint of an individual program task, but degrades much more slowly and much more linearly as the load increases,” he says.

“The mainframe isn’t necessarily a better tool than a PC server, or cluster of servers. It is a different tool and offers new possibilities to meet a class of application needs that aren’t well addressed by the traditional Linux hardware platforms,” he adds.

Daisy Whitney is a freelance writer based in Denver. Her work has appeared in the Denver Post, Electronic Media, and other publications. She can be reached at

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