Integrate: OpenRack

After Windmill and Watermark, the time was right another round of consolidation, bringing the rack design into the mix.

Dragonstone server renders

An issue with the Freedom servers was that the PSU was non-redundant (resulting in the Dragonstone design) and regularly had a larger power capacity than ever would be needed, which popped up on Facebook's efficiency radar. Adding another PSU in every server would mean an increased CAPEX and OPEX, because even when using an active/passive mode, the passive PSU is still using power.

A logical conclusion then was that this problem could not be solved within the server chassis, but could instead be solved by grouping power supplies of multiple servers. This resulted in OpenRack v1, a rack built with grouped power supplies on 'power shelves' supplying 12.5V DC for a 4.2kW 'power zone'. Each power zone has one power shelf (3 OpenUnits high), a highly available group of power supplies with a 5+1 redundancy feeding 10 OU (OpenU, 1 OU = 48mm) rack units of equipment. When power demand is low, a number of PSUs are automatically powered off allowing the remaining PSUs to operate much closer to their optimal points in the efficiency curve.

Another key improvement over regular racks was the power distribution system, which got rid of power cables and their need to be (dis)connected each time a server is serviced. Power is provided by three vertical power rails called a bus bar, with a rail segment for each power zone. After fitting each server with a bus bar connector, you can now simply slide in the server, the connector hot-plugs into the rail at the back, done. An additional 2 OU equipment bay was placed at the top for switching equipment.

Server Generations Mass Storage
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  • Black Obsidian - Tuesday, April 28, 2015 - link

    I've always hoped for more in-depth coverage of the OpenCompute initiative, and this article is absolutely fantastic. It's great to see a company like Facebook innovating and contributing to the standard just as much as (if not more than) the traditional hardware OEMs.
  • ats - Tuesday, April 28, 2015 - link

    You missed the best part of the MS OCS v2 in your description: support for up to 8 M.2 x4 PCIe 3.0 drives!
  • nmm - Tuesday, April 28, 2015 - link

    I have always wondered why they bother with a bunch of little PSU's within each system or rack to convert AC power to DC. Wouldn't it make more sense to just provide DC power to the entire room/facility, then use less expensive hardware with no inverter to convert it to the needed voltages near each device? This type of configuration would get along better with battery backups as well, allowing systems to run much longer on battery by avoiding the double conversion between the battery and server.
  • extide - Tuesday, April 28, 2015 - link

    The problem with doing a datacenter wide power distribution is that at only 12v, to power hundreds of servers you would need to provide thousands of amps, and it is essentially impossible to do that efficiently. Basicaly the way FB is doing it, is the way to go -- you keep the 12v current to reasonable levels and only have to pass that high current a reasonable distance. Remember 6KW at 12v is already 500A !! And thats just for HALF of a rack.
  • tspacie - Tuesday, April 28, 2015 - link

    Telcos have done this at -48VDC for a while. I wonder did data center power consumption get too high to support this, or maybe just the big data centers don't have the same continuous up time requirements ?
    Anyway, love the article.
  • Notmyusualid - Wednesday, April 29, 2015 - link


    In the submarine cable industry (your internet backbone), ALL our equipment is -48v DC. Even down to routers / switches (which are fitted with DC power modules, rather than your normal 100 - 250v AC units one expects to see).

    Only the management servers run from AC power (not my decision), and the converters that charge the DC plant.

    But 'extide' has a valid point - the lower voltage and higher currents require huge cabling. Once a electrical contractor dropped a piece of metal conduit from high over the copper 'bus bars' in the DC plant. Need I describe the fireworks that resulted?
  • toyotabedzrock - Wednesday, April 29, 2015 - link

    48 v allows 4 times the power at a given amperage.
    12vdc doesn't like to travel far and at the needed amperage would require too much expensive copper.

    I think a pair of square wave pulsed DC at higher voltage could allow them to just use a transformer and some capacitors for the power supply shelf. The pulses would have to be directly opposing each other.
  • Jaybus - Tuesday, April 28, 2015 - link

    That depends. The low voltage DC requires a high current, and so correspondingly high line loss. Line loss is proportional to the square of the current, so the 5V "rail" will have more than 4x the line loss of the 12V "rail", and the 3.3V rail will be high current and so high line loss. It is probably NOT more efficient than a modern PS. But what it does do is move the heat generating conversion process outside of the chassis, and more importantly, frees up considerable space inside the chassis.
  • Menno vl - Wednesday, April 29, 2015 - link

    There is already a lot of things going on in this direction. See
    and especially their 380V DC white paper.
    Going DC all the way, but at a higher voltage to keep the demand for cables reasonable. Switching 48VDC to 12VDC or whatever you need requires very similar technology as switching 380VDC to 12VDC. Of-course the safety hazards are different and it is similar when compared to mixing AC and DC which is a LOT of trouble.
  • Casper42 - Monday, May 4, 2015 - link

    Indeed, HP already makes 277VAC and 380VDC Power Supplies for both the Blades and Rackmounts.

    277VAC is apparently what you get when you split 480vAC 3phase into individual phases..

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