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Comment by dralley | Hacker News Reader

>This creates a single point of failure, trading robustness for efficiency. There's nothing wrong with that, but software/ops might have to accommodate by making the opposite tradeoff.

I'll happily take a single high qualify power supply (which may have internal redundancy FWIW) over 70 much more cheaply made power supplies that stress other parts of my datacenter via sheer inefficiency, and also costs more in aggregate. Nobody drives down the highway with 10 spare tires for their SUV.

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A DC busbar can propagate a short circuit across the rack, and DC circuit protection is harder than AC. So of course each server now needs its own current limiter, or a cheap fuse.

But I’m not debating the merits of this engineering tradeoff - which seems fine, and pretty widely adopted - just its advertisement. The healthcare industry understands the importance of assessing clinical endpoints (like mortality) rather than surrogate measures (like lab results). Whenever we replace “legacy” with “cloud”, it’d be nice to estimate the change in TCO.

DC circuit protection is absolutely not harder than AC. DC has the advantage in current flowing in only one direction, not two

Which makes it much harder to break the circuit vs AC

At 48 volts arcing shorts aren't the concern.

No one drives down the highway with one tire either.

Careful, unicyclists are an unforgiving bunch.

Let's say your high quality supply's yearly failure rate is 100 times less than the cheap ones

The probability of at least a single failure is 1-(1-r)^70.

This is quite high even w/out considering the higher quality of the one supply.

The probability of all 70 going down is

r^70 which is absurdly low.

Let's say r = 0.05 or one failed supply every 20 in a year.

1-(1-r)^70 = 97% r^70 < 1E-91

The high quality supply has r = 0.0005, in between no failure and all failing. If you code can handle node failure, very many, cheaper supplies appears to be more robust.

(Assuming uncorrelated events. YMMV)

Yeah but the failure rate of an analog piece of copper is pretty low, it'll keep being copper unless you do stupid things. You'll have multiple power supplies provide power on the same piece of copper

TL/DR, isnt there a single, shared, DC supply that supplies said piece of copper? Presumably connected to mains?

Or are the running on SOFCs?

The big piece of copper is fed by redundant rectifiers. Each power shelf has six independent rectifiers which are 5+1 redundant if the rack is fully loaded with compute sleds, or 3+3 redundant if the rack is half-populated. Customers who want more redundancy can also have a second power shelf with six more rectifiers.

I'm going to assume this is on 3 phase power, but how is the ripple filtered?

Inductors and capacitors usually

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