Slashdot Mirror
Data Centers And DC Power
mstansberry writes "In the final article in a series on the price of power in the data center, IT pros weigh the pros and cons of direct current-powered servers. A limited number of companies make servers with the power supplies removed with DC power distributed to multiple machines from a single unit. It saves power by skipping an extra conversion from alternating current (AC). Telcos have been using this method for years, but some data center pros are leery of taking on the new systems. It's not something people are familiar with and if they break down, you have to hire a specialized engineer to come fix them. But if they're saving even half of what they're reported to save on the electric bill, companies could afford to hire the engineers." We've reported on previous articles in the series.
I've always wondered (from a non-technical point of view) whether there was a benefit in having our homes wired up with two sockets (or maybe a 5 pin mains plug) giving standard AC voltage and a low-current DC voltage as well (12V?). So many devices only need low voltage, wouldn't we all benefit in having a power system in our houses in this way?
Jolyon
Please read my Canon EOS tech blog at http://www.everyothershot.com
ac goes into data centres, systems run on dc. Either it gets distributed to each computer as ac and converted in a medium-sized box in the back of each system, or it gets converted in one big box and distributed to the systems as dc.
The question is of the efficiency saving of doing all the converting in a big box against the efficiency loss of piping it around the data centre as dc, and wether you get a large total net saving (which I suspect that you do, since even inside the data centre, it's not going far)
FGD 135
trouble shooting and correcting DC power is simpler than working with linear power supplies. Unfamiliarity is the problem, not the technology.
The slashdot story intro implies that the advantage of DC is that you
save a conversion step. Well, maybe you do, maybe you don't, but
counting the number of AC-to-DC and DC-to-AC conversions is very
misleading.
Converting 50 or 60 Hertz to DC is much more costly and less efficient
than converting in either direction at a higher frequency. Low
frequency rectification requires large filter capacitors, complex and
expensive inrush current limiting, and active power-factor correction.
By doing that front-end work in one place only, preferably from a
3-phase source, you save power and increase reliability. You probably
still want multiple 50/60Hz to DC rectifier stages, of course, but now
they can be in parallel (for redundancy), rather than each one
downstream of the other where a failure of either one will bring down
the system.
Just because you're distributing DC to the racks, doesn't mean you
don't have to convert it again. It typically gets converted to AC and
back to DC at least once, usually twice before it reaches CPU and
memory chips. That's equally true in data centers that distribute AC
or DC. The fact is, memory and CPU devices want very low DC voltages
and very high currents. To make matters worse, not all parts of the
system want exactly the same DC voltage, you almost always have to
have multiple supply rails. You can't distribute very low voltages,
because it would require wires as thick as your arm and they'd still
be too resistive and inductive, so instead you distribute the DC at,
typically, 48 volts. The subsequent conversion to low DC voltages has
to happen via an intermediate AC, but it's a high frequency AC, so it
can be done much more efficiently using ferrite magnetic components,
active rectification, and often resonant mode filters. This high
frequency AC is confined to the internals of a power supply unit, it
never travels over wires or between boxes, thus reducing typical
high-frequency problems such as RFI.
I haven't mentioned battery-backup (i.e. UPSs). They make the system
more complex, but don't change any of the fundamental concerns. Even
on a DC distribution system, the UPS system requires it's own
additional stages of DC->AC->DC conversion, both while charging
(standby) and while discharging (during AC power failure). This is
because battery charging has to have a precisely controlled current
envelope. And batteries don't discharge at the uniform and
well-regulatted voltage that your DC distribution wants. They need
regulators, and switchmode regulators (typically DC->AC->DC) are the
most efficient choice.
The article on raised flooring was an interesting question, but stupid solutions.
That article talked just like some "Intelligent-Design" moron. Just because HE can't figure out how to properly model raised-floor airflow, it must not be possible to do it at all. Wrong. There are any number of companies that will do this for you.
The solution to raised floor airflow is proper modeling of the equipment, vent tiles, and blowers, and relatively unobstructed floor plenum. The solution is NOT air-cooled equipment on bare floor and overhead cable runs. If cooling is still a problem, then use liquid-cooled racks and equipment. (This is where things seem to be going right now.) While overhead cable runs may work fine for some dinky test lab, "real" equipment requires power cables of a size that would quickly fill most overhead runs.
This article proposing DC power is equally stupid.
An enterprise storage box, fully configured that I looked at requires 13,800 kVA of 208V three-phase power (100A inrush current). My mind can barely fathom the completely unbendable copper "wire" that supplying that much juice at 40-ish volts would require.
Telco's switches have a far lower power density than modern servers, and the DC power was made to correct for different problems.
If this guy's ideal data center is overhead cable runs, ceiling blowers, bare floor, and DC power, I'd run away fast.
SirWired
I think this article might be using the term "engineer" a bit too loosely. I doubt any company would hire an engineer - an actual person with a Professional Engineer's License - to work on these systems. A more appropriate term might be "technician," which usually refers to someone who is trained to repair and work with a single type of technology. Engineers, on the other hand, are usually trained to work with a large variety of technologies and usually work on either (A) Research and development, (B) Manufacturing, or (C) Failure analysis and redesign.
I guess using the term "engineer" sounds better though since it tends to scare the corporate fat-cats away from a technology because of the implied additional cost from hiring an engineer as compared to a technician.