While data center racks designed by Facebook and Google share many common principles – a shared power source for multiple servers in the rack is one example – two aspects have been radically different: server input voltage and physical depth of the rack. Facebook’s racks are 800 mm deep, while Google’s are 660 mm; Facebook’s input voltage is 12V, while Google’s is 48V.
When Google joined Facebook’s Open Compute Project earlier this year, however, the company said it would work on an open source rack design that would fit its needs and, hopefully, become standardized enough to where it would be readily available from a variety of vendors.
Last week, Google unveiled the first design document that resulted from those efforts, which it plans to submit for review to the Open Compute Project, the open source data center and hardware design effort founded by Facebook several years ago, whose members today also include Microsoft, Apple, Equinix, numerous major telcos and financial institutions, as well as nearly all major IT and data center infrastructure suppliers.
Google isn't the only OCP member besides Facebook that has developed their version of Open Rack. Fidelity Investments, one of OCP's early backers, submitted a design spec for its Open Bridge Rack in 2014.
Google's Open Rack 2.0 builds on the work Facebook and others have done for Open Rack 1.2, the latest version of the data center rack spec that’s been officially adopted by OCP. The biggest changes are of course additional specs for 48V power distribution and shallower racks to accommodate Google’s data center designs.
The latest spec uses modularity to address the fact that most companies using OCP designs probably aren’t going to switch to higher in-rack voltages and shallower racks any time soon.
The proposed standard includes two depth options: the original 880mm depth and the shallow derivation, which includes a shallow base rack and a modular extension for cable management and security provisions.
It accommodates two voltage options by interchangeable bus bars, which distribute power from centralized power shelves to IT devices. A rack with 12V bus bars can be retrofitted with 48V bus bars, but the bus bars aren’t connector-compatible, to prevent someone from accidentally installing 12V IT gear into a 48V rack.
While 48V power distribution is more efficient, according to Google, having both voltage options is nothing new for the company. It deploys 12V server trays into its 48V data center racks occasionally, but tries to avoid it whenever possible.
The reason providing 48V to the motherboard is more efficient is that it requires fewer conversion steps, since each step results in some energy loss. Facebook’s Open Racks are backed up by UPS systems whose output is 48V, which needs to be stepped down to 12V to accommodate its servers’ power requirements.
Google’s design feeds 48V to the motherboard, where power is stepped down individually for each component, such as CPU, memory, or disk. Urs Hölzle, senior VP of technical infrastructure at Google, said this difference makes for a 30 percent improvement in energy efficiency. The company has been using this 48V architecture for several years now.