Mainframes seem so last century to many but that hasn't stopped the mainframe division of IBM soldiering on -- and making quite a success of the business, if the company is to be believed. But many potential customers are eschewing big iron, and many existing ones are migrating away from it in favour of cheaper Unix-based platforms.

The New York Stock Exchange's data centre arm is no exception.

IBM makes much publicity mileage of its mainframe heritage -- and rightly so, since its big iron division introduced to computing technologies and concepts that continue in active and useful service to this day. Virtualisation is the most obviously popular right now, but the list includes caching and multi-processing, as well as the hard disk itself.

And the mainframe division of IBM -- still the largest vendor of big iron today -- trumpets the advantages the big boxes still have, including their fault tolerance and availability, their ability to crunch their way through vast batch jobs and -- a big plus in today's world -- their compute power per kilowatt. This has resulted in double-digit growth in customer numbers and growing, according to IBM's Mark Anzani.

Despite that, Francis Feldman, vice president of the shared data centre at Securities Industry Automation Corporation (SIAC), said that SIAC is moving away from the mainframe onto a Unix platform.

Feldman, who's in charge of the stock exchange's IT operation, explained the thinking behind SAIC's move -- and as you'd expect, it came down to money.

"It's about cost per transaction -- the ability to derive maximum throughput given the growth we're experiencing due to movements of models, regulatory requirements and share changes," he said.

"The mainframe is a phenomenal engine -- you get huge economies of scale. At 2,500MIPS the economies come back to you for some businesses but we're a 1,660MIPS shop, so we're below that mark. Project forward and it simply didn't seem to come together for us."

One of the arguments advanced by IBM for the mainframe is its ability to consolidate applications. Feldman reckoned he looked at a range of options before making the decision to switch to Unix.

"We looked at consolidation of applications within the mainframe. There were opportunities to use the mainframe as a database engine, we looked at aggregating other applications onto the mainframe, and at outsourcing the operation," he said.

"We investigated rewriting the applications -- could we mimic them on a different platform and reduce the transaction price? Also we looked at re-hosting -- using the same applications but changing the wrapper. So for example we might script them as opposed to using native JCL. We considered all the options."

But ultimately, it came down to cost: the mainframe was simply too expensive. How much did Feldman save by migrating to Unix?

"We will cut costs in half," he said. "And we haven't decided what to do with the box that we no longer have much use for."

Much of the saving came from increased application integration. "It's about the ability to share data, which becomes more compelling because it's all running under Unix or Linux, with no need for tricky operations such as ASCII to EBCDIC conversion," he said.

Also it's faster. "Our trading environment is client-server based and we platform-share with others, so we now get the speed of execution as the business demands it."

For Feldman, the mainframe is history compared to the new, low-cost standards-based Unix world. But he still sees a wider future for mainframes.

"The mainframe is a phenomenally architected platform. I suspect that for large shops with large applications there's a huge risk in switching. So as long as you can manage the bottom line and there's nothing new coming up, then the mainframe has a future," he said.

"But below that MIPS threshold, Unix is nipping at its heels."