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How important are redundant power supplies in servers, not measured by how critical server uptime is, but by likelihood of failure?

In other words, are server PSU's error prone and/or likely to fail within the first 3-4 years of service?

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Keep in mind that a redundant power supply gives you more then a spare power supply. With an additional power supply you can also get redundant power by plugging the two power supplies into different sources of power. –  Zoredache Dec 8 '10 at 17:30
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4 Answers

up vote 2 down vote accepted

I had a warranty PSU exchange in maybe 30% of my devices over the years. I consider this high enough that I want redundancy for this and would recommend it wherever feasible. In fact, PSUs were the only part except HDs I ever had to replace.

There are other reasons to consider redundant power as well:

  • An UPS won't protect you from a failing PSU. This might lead to broken filesystems etc, with unnecessary high recovery times.
  • You can connect your device to two or more different breaker circuits. Depending on your environment, you might not reach your server room fast enough before the UPS runs out of juice if a breaker trips. With redundant PSUs, hopefully the second circuit will last.
  • There are devices you cannot easily keep redundant, like switches etc. Here, a second PSU is mandatory, IMHO.
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One advanced feature available on high-end machines (especially servers) but also available to the general public for those willing to pay for it, is a redundant power supply . In essence, this is a power supply that actually includes two (or more) units within it, each of which is capable of powering the entire system by itself. If for some reason there is a failure in one of the units, the other one will seamlessly take over to prevent the loss of power to the PC. You can usually even replace the damaged unit without taking the machine down. This is called hot swapping, and is an essential productivity backup for use in servers and other machines used by a number of people.

Redundant power supplies are commonly used in conjunction with RAID arrays in systems requiring a high degree of fault tolerance.

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I think if you have multiple servers running (active-passive or load balancing mode), you don't need to worry about redundant power supplies. Running another identical server is better (in terms of availability) than having a complete redundant single server.

At the same time, it is recommended to have redundancy for your data using RAID.

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I disagree. While having redundant servers is a very good solution, I wouldn't recommend considering single servers "expendable", because that exposes you to a unnecessary risk when the other server fails as well, for whatever reason. This might not be true if you build a google like DC where many cheap systems are required, but this certainly isn't the norm. –  SvW Dec 8 '10 at 9:59
    
@SvenW: If you have a good power source, power failures will not occur so often. Also, journaling file systems will take care of the recovery process. For multiple failures, you need to fix the failing server as soon as possible by having a spare one ready to be plugged in. –  Khaled Dec 8 '10 at 10:19
    
Too many If's and When's. It's easy: Get a redundant PSU, reduce your risk exposure. It's not that expensive either. –  SvW Dec 8 '10 at 10:27
    
It is not a price issue. Your server box may not support dual-power supply. –  Khaled Dec 8 '10 at 10:37
    
Yeah, but the question is about if it is important to get a server with redundant PSUs. Is it. Very important. It would be nonsensical to discuss this if your existing servers doesn't support it. –  SvW Dec 8 '10 at 11:20
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Honestly, you're probably more like than not to have no issues with a single power supply over a span of 3-4 years.

That said, the power supply is one of the more likely pieces to fail in a server. Adding redundancy here is relatively cheap insurance against a failure. Having to order a replacement or drive out to get one can be a painful experience that one wouldn't like to repeat.

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