OAKLAND, Calif. — The complaints began two years ago.

"I started hearing from clients that they were running out of power in their data centers," said David Anderson, IBM's Green Evangelist for its System z mainframe. "It was a shocking thing."

Anderson envisioned a "data center of the future," where companies could boost computing power without using more energy.

The company took a big step toward this ideal Wednesday with the announcement that it would replace 3,900 servers with about 30 mainframes, cutting energy consumption by 80 percent and saving more than $250 million over five years.

"When you look at the mainframe, it's not only a viable alternative, it's in many instances the best cost alternative," Anderson told GreenBiz Wednesday.

The new global infrastructure, with System z mainframes running the Linux operating system, will support more than 350,000 users. In what it calls the most significant transformation of its data centers over the last generation, the company has worked to consolidate 155 data centers to seven colossal data centers. Six will participate in the initiative, which is part of IBM's Project Big Green announced in May.

The move will give the company about 85 percent savings in floor space. The company's eight million square feet of data center operations could fill 139 football fields.

Anderson called a mainframe a "server on steroids." It uses virtualization, where it acts like thousands of individual servers, to parcel out its system resources, such as processing cycles, networking, storage and memory. The System z9 mainframe can handle massive workloads. For example, it recently delivered a record 9,445 business transactions per second in real-time based on more than 380 million accounts with three billion transaction histories -- that is the world's largest core banking bencmark, IBM said.

IBM Global Asset Recovery Services will recycle the outgoing 3,900 servers. Trading them in for mainframes will save enough energy to power a small town, the company said. It also will save money on software because mainframes contain fewer processors, and software often is priced on a per processor basis. The company also expects to save time and money in system support.

IBM, Anderson said, faces no direct competition in the mainframe market. The company builds two models geared toward small to mid-sized operations, and for large-scale enterprises. Mainframe revenue has enjoyed growth for the past five consecutive quarters, the company said, and in 2006, mainframe revenue growth outgrew platforms based on the Microsoft Windows operating system.

Not everyone, however, needs a mainframe, Anderson said. Even those that use mainframes still use other servers, and not all workloads run on a mainframe.