As the list of arguably green technologies continues to expand, it's interesting to observe how many of the candidates are by no means new. Virtualisation has technically been around for years, for example, as have thin clients. But one of the more surprising technologies to make the list is the mainframe.

Yes, mainframes. Big Iron. Those big data-crunching behemoths regarded by some as the dinosaurs of the datacentre. Pundits have periodically predicted they'll go extinct -- or at least pondered aloud how long they'd be around.

But mainframes have continued to evolve -- and seemingly thrive. According to both IBM and IDC research, mainframes sales remain strong. And with green fever infecting the business world, Big Blue hopes that Big Iron will soon be regarded as Big Green Machinery.

As part of its billion-dollar Project Big Green endeavour, IBM revealed this week that it's partaking of a giant bowl of its own dog food, moving the workload of 3,900 of its 8,500 servers to 30 virtualised System z9 mainframes running Linux. (Yes, Big Blue says it will properly recycle those machines.)

"The cost of energy, power to run computers, storage, and networking equipment, as well as the power to the cooling equipment, is becoming the highest single cost of managing a datacentre," says David Gelardi, VP of industry solutions at IBM. "IBM took a look at these very interesting plums coming to the forefront at the same time. We have an opportunity with systems management tools, with Linux, and with virtualisation, to be able to take the workloads that are principally running on much smaller, under-utilised Unix servers and move them over to those 30 very large mainframes."

Big Blue anticipates the move from 3,900 servers to 30 mainframes will cut energy consumption by around 80 percent, a healthy cost savings no matter how you slice it. Specifically, the company anticipates reducing its total annual energy consumption, including power and cooling, from 3,266kW to 629kW, and total expenses from $2.86 million (about £1.4 million) to $551,000 (about £250,000).

Reports from Robert Frances Group (RFG) lend credence to IBM's claims that mainframes can deliver processing power more efficiently than standard servers. In a white paper titled "Mainframe Computing and Power in the Data centre" dated February 16, 2007, RFG reports the following:

"Mainframe systems consume less power, both in absolute and relative terms [than standard servers]. Typically, mainframe power densities are less than half of those of current rack and blade distributed systems. When looking at like workloads, the amount of energy consumed falls precipitously, in some cases the costs associated for power needed for an application are reduced by a factor of 600."

Part of this can be attributed to the fact that "mainframes are designed with a central AC/DC power converter, which operates at over 90 percent efficiency, compared with many existing rack server power converters which operate at 70 percent," according to a separate RFG report.

Also a boon: more precious floor space, moving from 11,045 square feet of occupied space to 1,643 square feet with the 30 mainframes.

Meet the machines

IBM's z9 mainframes are 64-bit machines, packing "specialty processors" designed for processing eligible Linux, Java, and data workloads as well as encrypting and decrypting certain data.

The machines' HiperSockets technology provides fast communication among all the virtual servers contained in a single machine, according to IBM. "By contrast, in a distributed environment, where many physical servers are connected by networking cables, lag time may be greater," the company argues.

Additionally, IBM says the machines can handle massive workloads. "The mainframe recently achieved the world's largest core banking benchmark result, delivering a record 9,445 business transactions per second in real time based on more than 380 million accounts with three billion transaction histories."

"Thank God for Linux."

Green ambitions -- both in terms of eco-friendliness and slicing energy bills -- are just part of the picture here. Linux's maturity stands to boost the mainframe's appeal, Gelardi says. "If you were to talk to just about any software company in the world, they would tell you the same story: "Thank God for Linux, because the 37 Unix variants were making us crazy. Linux is attractive because it's ubiquitous. You couldn't find too many products in the industry that don't support Linux."

Also appealing, according to Gelardi: the potential savings on software licences, which are generally sold on a per-CPU basis. Moving from 3,900 servers, which might have, on the low end, 7,800 CPUs, to the 30 mainframes, which represent, at most, 1,920 CPUs (64 per mainframes), means a substantial reduction in software bills.

Finally, IBM is making the fairly familiar case the having fewer machines means you can free up IT staff to work on other projects.

Geraldi stresses that the mainframe isn't suited for all server tasks, which is why the company isn’t trading in its remaining 4,000-plus servers for mainframes. Typically, mainframes have been used for bulk-data processing tasks such as ERP and financial transaction processing. "There are lots of workloads that will still favour other architectures. We do not today, and as far as I can tell, for the future, have this notion of this being one-sized fits all."