Thinking about redundant power supplies

October 5, 2005

The next time a salesman tells you that a server needs redundant power supplies for reliability, ask him if the server also has redundant CPU fans.

By now, most everyone understands that extended warranties on consumer electronics are just overpriced insurance (hence why the salescritter tries to get you to buy them, as they're great profit for the store). Redundant power supplies are getting to be a standard option in server configurations, and while they're not necessarily pointless I have to feel that a fair number of people are probably being sold ones they don't really need.

Buying a spare power supply is often a sensible thing. With redundant power supply designs, you are paying extra to have the spare plugged in and ready to go, instead of having to take some downtime to put it in. How much reliability do you actually gain, and how much is it worth?

Power supplies are not the only thing that can fail in servers; there's a lot of other moving parts, some of them under high stresses. Thus, having a redundant power supply may not actually be lowering the overall chance of system failure. (Or at least not lowering it by enough to change any important decimal points. The whole high availability field is an expensive minefield.)

Power supplies can fail in a number of ways; simply ceasing to supply power is the most benign one. With more explosive failures, there can be damage or problems outside the power supply itself, and your redundant power supply is unlikely to help. (It may even do harm, by keeping the machine running when an abrupt shutdown would better contain the damage.)

If total downtime is an issue (not just unplanned downtime), you need the power supplies to be hotswappable. Otherwise you'll have to take the machine down at some point to replace the failed power supply; the only question is when.

None of this is commodity PC hardware, which means that you're paying more in general. This is true even if you don't buy the redundant power supply; you're paying a markup on the base system for the mere possibility.

(So yes, this can mean that once you have to buy the base hardware for some other reason, you might as well get the redundant power supply; the extra cost may be entirely rolled into the base system price and the additional power supply itself not marked up.)

Comments on this page:

From at 2005-10-06 14:26:03:

I've only found one good reason for redundant (dual feed) power supplies.

I have machines which need to be up even when one of the building UPSes is taken down for maintenance. When that power work is being done, most often other UPSes remain available.

It saves reboots and time-critical rewiring to have the hosts and their storage arrays riding on two different power sources.

But of course I've never heard a salescritter mention that kind of reason. To them, redundant power supplies are just an extra bit of padding in their commission. The only story they know is 'reliability'. However, they usually can't give actual MTTF numbers to justify making the power supply redundant as opposed to making other components redundant at lower cost. Bleah.

From at 2006-01-05 06:07:08:

Redundant power supplies are sometimes installed to accomodate redundant utility power circuits. It's not always the power supply itself that fails, but sometimes the circuit feeding the power supply fails (breaker trips, battery dies, wiring problem, etc). In all the data centers I've worked in, multiple power circuits are available, served by independent UPSs and, in some cases, independent transformers, transfer switches, and generators. But even with redundant power circuits in the data center, it really only makes sense to install multiple power supplies in large, vertically-scaled machines like enterprise database servers. On smaller, horizontally-scaled machines such as web servers, which are often deployed behind load-balancers, we'll typically install a single power supply and put half the servers on one power circuit and half on the other.



By cks at 2006-01-06 03:01:29:

We're a university and therefor cheap. I'm pretty sure our biggest machine room has only a single UPS feed, although it's more than big enough to have multiple circuits. I'm pretty sure that even the most precious machines we have in the machine room are on only single circuits. (They probably have redundant power supplies, though.)

Written on 05 October 2005.
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Last modified: Wed Oct 5 03:09:01 2005
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