After months of prerelease code availability, IBM today announced the general availability of Linux for the S/390 mainframe platform. The company adds System/390 hardware to its list of supported environments for Linux, completing a range of systems that extend from Intel laptops to the largest mainframes the company has ever made.

IBM has for several months made the S/390 port of Linux available on a Marist College web and FTP server. IBM funds numerous leading-edge research projects and academic ventures under a long-standing partnership with the college, which is located in New York State. Over 2,100 S/390 administrators have downloaded the free software since it became available in January, and an IBM executive says the company has gotten the clear message that enterprise customers want Linux.

Linux for S/390 will be supported by IBM Global Services (IGS) right alongside the other mainframe operating systems, such as OS/390 and VM/ESA. IBM is also announcing partnerships with SuSE and TurboLinux to distribute Linux for S/390 and provide value-added support for the product. This partnership gives IBM the Linux credibility and expertise of TurboLinux and SuSE, while allowing these Linux-oriented companies to reach mainframe-oriented enterprise customers who might otherwise be inaccessible.

In addition to Linux itself, IBM will be releasing native versions of its key middleware and database products for Linux on S/390 as well, including the client "connector" for DB2 Universal Database.

In keeping with the Open Source distribution of Linux, free downloads of the S/390 port will continue to be available; customers will pay for support and services, or for add-on software, and not for Linux itself. An IBM spokesperson acknowledged the benefit IBM gains from partnership with Open Source developers, saying, "We need them even more than they need us."

IBM has earned both praise and criticism from the Open Source community in recent months for its Linux efforts. One portion of Linux for S/390, specifically the device driver for the OSA2 network adapter, is being released "OCO" (Object Code Only) by IBM, on the grounds that revealing the source code would allow competitors to copy their hardware design. On the other hand, IBM has released the source code to its Journaled File System (JFS) and Logical Volume Manager (LVM) from the AIX operating system so that these technologies can be deployed on Linux. The company has not publicly stated which other products, if any, will be released as Open Source code in the near future. All of the kernel and GNU C changes, however, are released as Open Source.

There has been cooperation between the Linux kernel team and IBM as well, with Alan Cox being a regular participant on the Linux for S/390 e-mail list at Marist College. The architecture-dependent features needed to support Linux for S/390 are being merged into both 2.2.x and 2.3.x kernel trees.

Boas Betzler, technical lead at the IBM Linux Technology Center, is enthusiastic about Linux and about working with the Open Source community. "We really liked the [Linux kernel] architecture," he says, referring to the way in which hardware-dependent parts of the kernel are segregated from the common code base. Betzler added that IBM's System/390 hardware happens to have features that map very well to the design model of Linux. The S/390's management of symmetric multiprocessing (SMP) in particular maps very well to the way Linux works, and Betzler says the IBM hardware actually implements a superset of the functions Linux needs.

When asked about the relationship between his team and the Linux development community, Betzler had only good things to say. "The Linux community in general, and especially Alan Cox, have been very receptive" to incorporating S/390 features into the kernel. He felt that there "might be more objections" if IBM had asked for the common code to be changed, rather than just the architecture-specific portion. Betzler believes Linux will benefit from the introduction of mainframe design principles, but says this is a long-term process which "we are only starting...to explore."

Linux for S/390 can run on bare metal, as it does on other machines, or it can run in a logical partition (LPAR), which is a hardware-supported way to subdivide the processor and memory resources of a System/390 machine. Perhaps the most versatile installation mode, though, is as a "guest" under IBM's powerful VM/ESA operating system. VM/ESA actually virtualizes the entire S/390 hardware and I/O environment to such a degree that each login session runs completely in its own virtual machine. Most end users have traditionally used CMS (a command shell) under VM, but VM is able to support just about any S/390 operating system as a guest. It is similar to the way VMWare works on PCs, but the hardware support is tremendously more efficient than the Virtual 8086 mode provided in Intel hardware. The result is that overhead for virtualization is miniscule, and it is practical to run hundreds or even thousands of virtual machines simultaneously. One site testing the prerelease of Linux for S/390 actually ran 41,000 instances of Linux at once, on a single S/390 mainframe with VM/ESA as the host operating system. No one recommends this as a practical limit for production, but it was quite impressive as a demonstration of what is possible.

In spite of its versatility and power, though, VM/ESA has been short-changed in IBM's marketing for years, according to some of its advocates. The reasons cited by people on the Linux for S/390 e-mail list vary; some point to internal politics, some to the revenue structure for mainframe software, which favors the OS/390 operating system. Others say that the cost of VM itself is simply too high and scares off executives who don't understand its true value. In any case, Linux for S/390 could turn out to be a boon for the VM advocacy camp. Internet and application service providers could potentially deploy an instance of Linux on S/390 for each customer account, without the need to buy, configure, and support separate PC servers.

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