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Was Thomas Edison Right about DC Power?
Declan McCullagh writes "Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla. But now DC power is being seriously considered for data centers. DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure. The University of Florida has even bought 200 DC servers."
Was Thomas Edison Right about DC Power?
:-)
Oh, well, nothing sensationalist about that headline. (*rolls eyes*)
DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure.
In this case they're right. With that much hardware that close together, it's easier to treat the entire room as a single device. As the article suggests, this cuts down on waste heat produced by inefficiencies in AC->DC conversion. In fact, it significantly cuts down on the amount of equipment needed in the entire room. The concept can be taken as far as to cutting down to a single power supply per rack.
The amusing part about this is that the resulting racks might look a lot like Big Iron servers with pluggable motherboards.
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I heard of this new power system. Seems like a mix of AC and DC, to create the ultimate power form. AC *lightningbolt* DC was the name, and with a lightning bolt in the name, it has to strike you like thunder.
Tesla and Edison were both right...and wrong. Like many Slashdotters do when debating which operating system is best for any given job, Tesla and Edison wanted to apply one power system to every job. Its like having a toolbox with only a screwdriver in it. Ever try to drive a nail with a screwdriver?
For moving power over long distances, AC is king. But for short distances with most modern electronics, DC would win. The first thing a desktop system or server does with AC is converts it to DC. So if you have a number of machines all in the same room, why not do the conversion in one spot, and eliminate the redundancy in every machine.
Would it benefit the average user with one or two machines? Not at all. But for a major center with many machines in the same room, I can see quite a bit of benefit with going with DC.
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Brilliant!
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for crap's sake, dc powered servers are nothing new, many have config option of "-48VDC standard telco" supply.
That said, if space and cooling are an issue it might well make engineering sense to get the transformers, capacitors, and rectifiers out of the computer boxes. Big 5v/12v power busses wouldn't even need to be insulated. So while the reporter badly mangled the story, the engineering sounds reasonable to me.
Actually, that's the norm across the phone industry. Everything, and I mean everything, runs on -48V DC. Okay, not the fluorescent lights....
This goes back to the telephone talk battery, which is -48 V DC. That powered the phones via old cord switchboards, and was the voltage of electromechanical (stepper, and later crossbar) switches, which basically used relays. Electronic gear was then designed to run on the same power plant. A telephone building has a big bank of batteries, powered by multiple "rectifiers" (DC supplies) which, btw, are normally engineered to not run over 40% of load. (That way they can still run the systems and recharge the batteries when one of them is kaput.)
If you then put anything else into one of their buildings, the Network Equipment Building Standards (NEBS), which are Telcordia documents that practically carry the force of law, dictate that equipment be DC powered. Among other things -- NEBS gear has to meet the brick schytthaus test. (Sun Netras and many Cisco routers meet NEBS. Your basic rack server doesn't. And aluminum racks are STRICTLY forbidden; it has to be steel.)
So because of the talk voltage on analog phones, lots of computing equipment is engineered for -48 V DC power. Sort of like the legend (I know, that one is not really true) about the railroad track gauge being based on Roman chariots. But in this case it's surprisingly effective.
Wrap the casket in copper, replace the headstone with a magnet, and expose corpse to this article. As Tesla turns in grave, free power.
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The origin of the 48 volt number is that it was convenient, and now it just sneaks under the 50-volt "low voltage" cutoff in the NEC, which I think was written with telcos in mind. The glorious thing about this is that you don't need licensed electricians to do power wiring in a central office.
And the reason it's negative with respect to ground goes all the way back to the telegraph system: Western Union initially ran bipolar lines and noticed that the positive ones corroded much faster. Sodium ions (from dissolved salt) are negative, and thus repelled from lines that're also negative. The whole phone system was built with positive ground because of this, and it's saved incalculable maintenance costs. It does tend to mess with people's heads the first time, if they're used to negative ground systems, but you get over it quickly. (A number of traditions use blue for "hot" and black for ground/return, to help escape your "red equals positive" association.)
DC power as used by telcos is also always redundant. There's an A-side and a B-side for everything, and the cables are sized so that the entire load can run from just one side. This leads to some very fat copper, which is cheap compared to downtime. You don't achieve five-nines reliability with a system that contains single points of failure!
Now, about rack-mounting: This was also invented by the telcos, originally in a very wide (40-inch?) format, for the panelboards and Strowger switches. Some of the old crossbar equipment is still in those huge racks, but the 23-inch width is infinitely more common now. All telco equipment is mid-mounted, with the ears approximately in the center of gravity on the shelf, so the force on the screws is shear. There's no torsion on the mounting flange unless you step on the front or back of the shelf. Cooling is always convective bottom-to-top, or occasionally front-to-back with fans. This leads to a "cool" front aisle and a "warm" back aisle between alternating rows of equipment.
Now, the pro audio industry borrowed the rackmount idea fairly early on, but they were mostly mounting control panels and mixers, which are very shallow, so flush-mounting made sense. They also changed the every-inch Western Electric mounting holes to an alternating-spaces "EIA" standard, and narrowed the rack from 23 to 19 inches.
Somewhere along the line, an absolute idiot decided that computers should be rackmounted, but they should be 19 inches wide, flush-mounted, and use EIA hole patterns. I'm sure this has something to do with mainframe legacy getting perverted by peecee people. The current mishmosh of mounting standards (19" vs 23", two-post versus four-post, flush versus mid, inch versus RU, front-cable versus rear-cable) is what every datacenter tech deals with on a daily basis. Throw overhead racks versus raised-floor cabling into the mix, and you've got a recipe for frustration!
If you're familiar with the concept of "blade servers", where common components are separate from processor resources in the shelf, congratulations. Telco hardware has been built like this since the invention of the circuit board. Actually, the concept of replacable plug-in units goes back before that, but it got vastly easier with printed wiring boards and card-edge connectors in the sixties. Most of the "good ideas" in serious computing circles are actually century-old ideas in the telco industry. Spend a week shadowing a central office tech before you design a datacenter, please!
Also consider: If your datacenter is already built for DC, throw some solar photovoltaic panels on the roof. Inverters are a large part of most PV systems' expense, and you can skip that part. Why not start offsetting your grid demand now?
Also also: Edison was flat-out wrong about DC. The modern switching power supplies that make DC transmission lines practical didn't exist in his day. Besides, long-distance power transmission is an entirely other discussion.
The article conflates several things.
First off: Digital electronics generally requires several voltages. And they're all low, requiring high currents, massive conductors, and local filtering and regulation. So even if you're providing DC power from outside the room, you'll have a switching power supply (or several) in each piece of equipment to convert whatever the rough DC power is to whatever you need, smooth it, and regulate it.
But while some electronic devices use a common switcher to generate all the voltages with one conversion step, others use a "roughing" supply and a bunch of local supplies. Part of that is to get better regulation - part is because the roughing supply must run from 60 (or 50 or whatever) Hz and thus requires big caps to tide you over the low part of the cycles - caps you don't want taking up space near the components.
If you're going to do it in two stages anyhow, you can put your roughing supply OUTSIDE the room and only have the final supplies inside. The roughing supply has a lot of heat dissipation so you save a bunch on your cooling.
Second: There are two standards for power distribution in electronics rooms:
- Your local power line stuff. (120/240/480/208-3-phase in the US)
- The telco standard: x2-redunant 48V DC.
A lot of equipment - especially networking equipment - is manufactured for sale to tellcos and other operations that use the standard. They might have initially used it because some of their equipment was co-located in tellco sites, where only 2x48VDC is available - and they got a quantity discount for buying a bunch of the same stuff and went to 48V for their own sites. Or they might use it because it's MUCH simpler to do backup power with floating batteries and century-old technology than with a building-sized UPS. (Note that a UPS CAUSES at least one outage when first installed and on the averate at least one more within the first year of operation from some malfunction. And a UPS dissipates more power than a roughing power supply or a battery charger.)
But the standard for 48VDC is REDUNDANT 48VDC supplies, with the equipment only requiring one (and typically doing "cutover" with diodes B-) ). With the equipment already set up for redundant supplies it's not a lot of cost or work to wire both sides and put in two 48V feeds to the equipment room. (Four diodes are a LOT cheaper than a pair of 120V roughing power supplies at each box, too.) So of course the users of such equipment normally give it dual supplies. (Even if it's a single rack and so they just put two roughing supplies in the rack fed from two different 120V feeds.)
The result is that all the equipment has redundant power supply, and keeps operating glitch-free through a number of kinds of partial outages - AND power supply repair and replacement. This is what's responsible for much of the claimed increase in reliability.
The whole Edison/Tesla DC/AC war had to do with the economics of CROSS-COUNTRY power transmission. AC beat DC there because a century or more ago it was virtually impossible to jack DC voltages up to levels suitable for long-distance transmission and back down to levels safe for distribution within houses, while AC could do that easily and efficiently. So Westinghouse/Tesla could ship cheap power from Niagra Falls to New York City while Edison had to build fuel-burning power plants IN the city. It has essentially nothing to do with shipping the power around within a single building.
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Tesla originally worked for Edison, but they had a bit of a falling out, which is possibly why the AC/DC competition was so heated. Edison embarked on a pretty ruthless and gruesome campaign to discredit AC power, at least by modern standards. He electrocuted stray dogs and cats with AC current in public demonstrations intended to show how dangerous AC power was.
In one instance, he even electrocuted an elephant...
During the construction of Luna Park on Coney island, an elephant used as a beast of burden went out of control and killed a couple of people. Topsy, as she was called, was condemned to death. However, there was a wee bit of a problem. Elephants aren't the easiest critters to kill. What happens if you walk up and fire a shotgun at it's head, only just to piss if off? They do have rather thick hides, and we are talking about a homicidal elephant the size of a couple SUV's here. There weren't any cliffs handy to stampede poor Topsy off of, and I doubt dynamite was ever seriously considered. Edison, being the generous person he was, gladly volunteered to execute the elephant with AC current and filmed the whole thing. He showed the resulting film, "Electrocuting an Elephant" (1903) publically on many occasions. It is quite probable that many a cat and dog escaped a crispy fate thanks to this film. If you decide to track down a copy of "Electrocuting an Elephant" today, please be warned that it's a rather gruesome little piece of history, and is not for the faint of heart, or SPCA members.
Edison and Tesla were both right. Remember, the DC vs. AC wars were fought back when the load was mostly made up of lights, motors, very utilitarian things. AC is fantastic for transmission over long distances (and for running three phase motors, but that's another story). DC happens to be better at running precision equipment like computers -- heck, they all run on DC already. All we're really talking about here is taking advantage of an economy of scale by doing one big power supply (or a few, for redundancy) instead of one for each machine.
Ever seen a telco rack? Everything runs on -48VDC. Everything. A telco rack always includes a couple of DC power supplies, and all the equipment just ties in to a common DC bus. The best part of all: the UPS simply consists of four "car batteries" (not exactly, but you get the idea) wired in series and tied directly into the bus! No pesky inverters to deal with.
The telecom industry has been doing it this way for decades. It's about time the computer industry got on board.
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