Big Blue signed the death warrant for OS/390 when it introduced a new version of its z/OS flagship mainframe system last week.
Alongside the z/OS upgrade the company announced plans to stop selling OS/390 on 17 December 2002. It also warned it would end OS/390 support from September 2004.
The move could leave some users having to make difficult decisions in a hurry. Organisations running IBM's low cost 31-bit G5 and G6 family of mainframes that run OS/390 must decide whether to freeze their current G5/G6 installations or start a migration on to z/OS, which supports 64-bit computing.
Gartner analyst John Phelps said the decision would hinge on predicting future capacity and functionality demands.
If an organisation running G5 or G6 equipment was confident its mainframe systems could meet current and projected capacity requirements, it should be safe for the next 24 to 36 months, according to Phelps.
However, if an organisation's capacity demand was likely to increase, or it required z-based functions, the only option would be to upgrade from OS/390 to z/OS.
Any upgrade to a mainframe system can bring major business disruption, warned Phil Payne, an analyst at Isham Research. "It is a massive job for companies to bring their machines down and test a new operating system release," said Payne. Some mainframes are only rebooted every couple of years, he added.
Julie-Anne Williams, chairman of the large systems working group at Guide Share Europe, an IBM user group, said users running OS/390 Release 10 should apply an IBM software patch to upgrade to z/OS, while installations on earlier versions of the OS would need to load a service pack release.
Either way, it was a significant undertaking. "You can put the OS [update] on a single box in about five days but in a production environment it can take from a matter of weeks to many months to test it fully," she said.
Analyst firm Meta Group estimates that since it was introduced 17 months ago only 10% to 15% of IBM mainframe users have migrated to z/OS.