Now, there are signals of another seismic shift: a growing trend towards selling mainframes that run Linux and the z/VM virtual machine software, without IBM's traditional z/OS mainframe operating system.
That's "a dramatic change," says mainframe consultant David Boyes, "which is changing the whole direction of that part of the company."
The change in attitude may stem in part from Linux' role in breathing new life into the moribund mainframe platform, which saw little or no sales growth in the 1990s.
Today, however, IBM is seeing "enormous growth on Linux on the mainframe," says Colette Martin, IBM's zSeries product marketing program director. In 2001, she says, Linux accounted for about 11% of the new mainframe MIPS IBM sold. Last year, that figure jumped to nearly 20%.
And while Linux-only mainframes are currently only a small part of that, IBM "is certainly seeing a trend" in that direction, according to Martin.
Big Blue's first Linux-only mainframe, the z800 model 0LF, debuted just over a year ago. In the year since it was introduced, says Martin, the model 0LF has made up perhaps five percent of z800 sales.
The z800 is IBM's entry-level mainframe. Pricing for the z800 series starts at around $200,000; the Linux-only model is sold as a package, including three years of maintenance and z/VM, IBM's virtual machine software, and starts at around $350,000.
IBM says it does not support its larger z900 series mainframes -- which start at around $750,000 -- using only Linux, although some customers, such as Finnish telco Sonera, are reported to be running z900s using only Linux and z/VM.
There are three possible ways to run Linux on a mainframe, according to Jens Sandmann, enterprise sales manager for SuSE. Most common is to run Linux on a mainframe which is running z/OS, either in a logical partition, or LPAR, or on a hardware engine dedicated to Linux. IBM's IFL -- Integrated Facility for Linux -- engines, which run only Linux, cost less than processors which run z/OS.
Other IT shops run Linux and z/VM without z/OS, as does the z800 model 0LF. IBM's z/VM is virtualization software which allows dozens or hundreds of separate Linux instances to run on the same mainframe.
According to many industry observers, z/VM is one of the keys to the success of Linux on the mainframe, because it allows IT shops to easily consolidate many smaller servers onto one mainframe. It also greatly eases system management, since Linux virtual machines can be created or deleted purely in software.
It is also possible to run Linux on the mainframe's bare hardware. That simply creates one very large Linux server, which is not an optimum use of the system, according to Rich Smrcina, a Linux mainframe expert at mainframe consulting shop Sytek Services.
Very few, if any, IT shops appear to be running in Linux mainframes this way. "It would be a very expensive solution," says Smrcina.
From server consolidation to Cobol
Linux on the mainframe is well suited for applications such as consolidating large numbers of smaller file, print or mail servers on one system, according to SuSE's Sandmann. It's also well suited to ISPs, who can give customers their own virtual Linux server, reassuring them in terms of security.
At least two large ISPs in Europe, Finland's Sonera and Sweden's Telia, are using Linux mainframes for this purpose.
Other early adopters of Linux-only mainframes are using their systems for completely different types of applications.
The Electrical and Computer Engineering department at the University of Florida at Gainesville, for example, is building a Linux-only z800 as the heart of a research project in grid computing.
And the German firm ABK Systeme GmbH, which manages payment transactions for banks, is buying one to run a legacy banking application, which is written in Micro Focus Cobol.
Head to head with Sun and HP
Linux-only mainframes, says Boyes, gives IBM an opportunity to sell into IT departments that are not mainframe shops, and have no in-house z/OS or MVS expertise.
These are shops that "don't have a mainframe. They gave up JCL [IBM's mainframe Job Control Language] long ago, and they don't want a z/OS system," he says.
This puts IBM squarely "head to head with Sun and HP," says Boyes. "These are exactly the customers that Sun and HP market the Sun Fire 15K and the SuperDome to."
Even though the large Unix systems can often provide more raw CPU horsepower than mainframes, users are turning to them for their overall throughput capabilities, as well as their reliability.
ABK Systeme was running on Unix systems, but chose a Linux z800 mainframe because it wanted the capacity to handle rapid transaction growth, says Armin Gerhardt, the company's CEO.
The mainframe's much-vaunted reliability was also important to ABK Systeme. "With a mainframe," Gerhardt says, "you don't worry about things like whether you've got mirrored disks or multiple processors in case of CPU failure. And it's not like a PC, where you have to reboot it to get it running again. A reboot is not something you need to think about in a mainframe data center."