The result is this: a pretty impressive piece of hardware, with 640 real cores (or 1280) if you count HyperThreading.
The hacker is going by the name of Steve, and posted pictures of his array on the SimBimbo wordpress blog, as well as answering many questions.
He had to design his own shelving as nobody makes a 1U rack capable of holding four Mac minis. Steve worked with a vendor to design his own custom rack that could hold the Mac mini devices, and each shelf has its own special plastic insert that ensures air flows into the Mac minis, and not into the intake of the adjacent mini.
He also designed custom 'One To Four Y' power cables for the Mac minis, so that each rack of four computers can plug into a single outlet. He also built a cusomter cooling door made out of four car radiator fans, connected toa 40A DC Motor Controller.
The final result is a professional looking rack that can replace an Xserve.
Apple first started making the Xserve server in 2002, but stopped production in 2010 and stopped accepting orders in January 2011. Steve Jobs, former Apple CEO, said in an email communication to a customer that "hardly anyone was buying them". A Gartner report suggested that Apple was selling in the region of 10,000 Xserve units per quarter, a tiny amount for a company with such high profile products as the MacBook, iPhone and iPad.
Apple now sells Mac Pro and Mac mini computers with Mac OS X Server software instead. This has left server administrators looking to continue running a servier based on Mac OS X software in something of a quandry, as it's not normally that easy to integrate desktop computers into a server environment.
Steve's server rack looks like a good solution. There's no mention of him putting it into production, however. On his blog he says: "The company I work for requires large numbers of machines to build and test the software products we make, these products support Windows, Linux and Mac so we have data centers with thousands of machines configured with all 3 OS’s running constant build and test operations 24 hours a day 365 days a year. This is just a small look at the Mac side of things."
There's no information on how effective the server rack is, Steve says: "These machines aren’t servers, they serve nothing, they are treated as individual machines. They are all managed and allocated by specialized “Cluster” software that allows us to run our Build and Test functions. As for device failure, we treat these machines like pixels in a very large display, if a few fail, it’s ok, the management software disables them until we can switch them out. This approach allows us to continue our operations regardless of machine failures."
This isn't the end of the rack for servers, however, when asked why he didn't mount them vertically, Steve said: "I tried the vertical approach, but manufacturing the required plenum to keep the air clean to the rear machines cost too much for this project, but it’s not off the table for the next rack." The next one will be bigger too: "we are going to begin working on a solution to fit 6 mini’s per 1U of space, resulting in a rack with 240 Mac Minis in it. I can’t wait to start working on that in January. People thought I was crazy building this. Wait till they see the next one."