Computerworld - When IBM stopped selling its OS/390 operating system last December, the clock started ticking for users of the mainframe system to switch to the newer 64-bit z/OS. Users who delay making that transition could find themselves marooned on an unsupported platform in the not-too-distant future, users and analysts say.
This isn't a big surprise. "IBM has been very aggressively informing and reminding people about this for a number of years," says Mike Kahn, an analyst at The Clipper Group Inc. in Wellesley, Mass.
IBM released z/OS in October 2000, along with new 64-bit zSeries mainframe hardware. Already, IBM is into its fourth version of z/OS and poised to phase out support for OS/390 technology. For example, support for OS/390 Version 2.10—which is the last OS/390 release—will cease in September 2004, while support for Version 2.09 will end this month.
The company has for some time now been telling OS/390 users to move to its latest release, z/OS 1.4, not only to ensure continued support, but also because IBM will use this release as a base for future hardware enhancements.
In addition, z/OS 1.4 is the last release of z/OS that OS/390 users will be able to migrate to in a single step. Moving to future z/OS releases from OS/390 will involve a two-step process.
Analysts estimate that more than 80% of mainframe workloads are currently running on z/OS. However, the machines running OS/390 far outnumber zSeries systems because the OS/390 machines tend to be much smaller, says Phil Payne, president of Isham Research in Great Stukeley, England. "Numerically, [OS/390 machines] account for 80% of the mainframes out there, but they don't account for much of the power," he says.
The difficulty of the migration task depends on the release of the operating system and the hardware that users are currently on, say users.
For instance, users who are on OS/390 Version 2.10 can move to z/OS 1.4 and continue running the operating system and all applications in 31-bit mode on their existing 31-bit System 390 hardware.
For such users, the migration really is no different from moving to any new operating system release, says David Danner, a Washington-based consultant who recently helped a large government agency migrate to z/OS.
For the many users who aren't ready or can't afford the move to a full 64-bit environment, which involves buying expensive new zSeries hardware, running z/OS in 31-bit mode on S/390 hardware should do nicely, users and analysts say.
The University of Florida's data center in Gainesville, for instance, has already migrated to a 31-bit z/OS environment on its existing S/390 mainframes. "It is current code, and it allows you to keep up with new functionality as you need it," says John Bevis, associate director of the data center.
High Costs of Upgrading
But for the moment, the university has no plans to switch to 64-bit mode on z/OS because of the high upgrade costs of software licenses and hardware. "The processor and software costs can kill you," says Bevis, a former president of Share Inc., an IBM large-system user group.
"I don't think 64-bit [functionality] is what's driving some people. It's support and continued maintenance," Clipper Group's Kahn says.
Users who want to take advantage of full 64-bit functionality need to prepare well for the transition from a 31-bit environment, Danner says. Code and applications that work well in a 31-bit mode don't always migrate easily to z/OS, says Eric Bielefeld, a senior MVS systems programmer at P&H Mining Equipment, a unit of Joy Global Inc. in Milwaukee.
"We had problems with a buffering product and a data-compression product," Bielefeld says. Fixing the problems required intensive work with the vendors involved. "It took me a good six months to install and test everything," says Bielefeld, whose company moved to a z/OS environment last October.
Users on releases of OS/390 prior to 2.10 "face a whole bunch of change all at once," Danner says, including moving to z/OS in multiple stages.
For its part, IBM has done a lot to ease the migration to the 64-bit world, users and analysts say.
A "bimodal migration program," which allows users to switch between 31- and 64-bit mode, has been particularly useful, says Bielefeld. With it, users can move applications over to a 64-bit z/OS environment and, if problems arise, move them back to 31-bit mode while those problems are fixed.
IBM has also restructured its pricing models to minimize the software upgrade costs associated with moving to z/OS. And IBM handbooks and other materials that provide detailed technical help are available.
A Choice of Two Migration Paths
Users running OS/390 Version 2.10 on S/390 hardware have two routes to the 64-bit z/OS: 
Stay with their hardware but move to 31-bit z/OS Version 1.4. Then move to either z/800 or z/900 hardware and run z/OS in full 64-bit mode, with the option of switching back to 31-bit mode for the first six months. Continue running OS/390 Version 2.10 in 31-bit mode, but move to z/800 or z/900 hardware. Then switch to 64-bit OS/390 Version 2.10 and then to 64-bit z/OS.
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