As far as I understand, computers don't always draw the same amount of power from the power supply all the time. There are times when hard drives are on standby and not using as much as when they're spinning and graphics cards save power when not fully utilized.
What happens when, for example, you have 100 hard drives installed in a desktop tower (or a server rack) with let's say 1000 watt PSU, and they're all on standby, and then suddenly some process accesses all of the hard drives and spins them up, drawing more power than PSU can give?
Is there some signal that hard drives send when they think they're not getting enough power? Or does each individual hardware piece ask the PSU if it can provide it X watts of power, and it may say "no, I don't have that available"? Does the motherboard decide if it can negotiate this power request and safely avoid sudden power loss and instant shutdown? Or is the standard protocol in this case to drop dead without trying to avoid this problem?
From my experience with my desktop and a few hard drives and a low power 350W PSU, it would instantly shut down if 5 hard drives were all trying to spin up at the same time. Nothing bad happened, fortunately, but I'd like to know if instant shutdown is an expected and planned reaction of hardware pieces, or just the motherboard (or PSU) freaking out and disabling everything unexpectedly.
To clarify my question: What I'm interested in is why the common result is system shutdown instead of a safe denial of power to the device which would overload the system? USB power management protects against such a scenario, so why doesn't SATA/Molex power cable management logic not have this (or if it does, why it so commonly fails)?
Update after seeing some answers: I'm really surprised there isn't some sort of power management logic built into PSUs like motherboards have for managing USB power distribution. That's what I got from the answers so far. If you know something that says otherwise, please share as an answer.
Best Answer
From an electronics standpoint, once the current drawn from a supply exceeds the supply capacity then the output voltage will suddenly drop. The electronics that requires a particular voltage to work will simply turn off. This is effectively a power brownout.
In the best case, the power supply detects this overload condition and holds itself off for some period of time or tests to see if the load is still there in a safe fashion, holding the power output off until the load is gone.
In the worst case the power supply goes into constant turn-on and brownout cycles and potentially kills itself or one or more of the devices connected.
There is no provision for devices to "request" more power from a power supply except in intelligent devices like USB where power availability was a concern to begin with. A system power supply has absolutely no intelligent electronics.