I have 6 servers both with 2 power supplies attached to an UPS. I want to add 4 more servers but the UPS is full and due to budget restriction I couldn't buy a new one for months. The 6 + 4 new servers are running essential services (high uptime required).

Questions :

  • What is the power supply failure rate or how many failure have you experienced so far ?
  • If the failure rate is low, how safe is it to use only one psu per server and keep the redundant one as backup if the first one fails ?
  • Does it make sense to connect one psu to the UPS and the second one to a normal plug ?
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    If they're running essential services then you can afford a UPS. Take the chance of downtime, multiply it by the amount that downtime will cost and show it to a manager. – WheresAlice Feb 14 '10 at 16:20
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    You can't have high up-time with thinking like that. If budget is the problem it might have made more sense to buy 3 servers and 1 UPS now and another server later. – John Gardeniers Feb 14 '10 at 20:47

If all the servers have redundant PSUs (), you can plug just enough PSUs into the UPs and rest into the main powergrid. If you have 10 servers, with dual PSUs each (which makes 20 PSUs to plug into the power), you can simply plug only one of them into the UPS (so you would only take up 10 sockets in the UPS's PDB) - in case of power failure, the UPS will hold the servers up.

However, this does not cancel the previous comments about a UPS not being the ultimate answer to power failures.

  • 10 servers, 5 sockets? I'm having trouble with the arithmetic on this one. – John Gardeniers Feb 14 '10 at 20:46
  • ouch, me too :) you're right, obviously, and I need to start drinking coffee on Sundays. – dyasny Feb 14 '10 at 21:03
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    In general, why bother connecting dual, redundant power supplies to a single, non-redundant UPS? I believe that the highest probability of failure is neither the main grid nor the UPS, but the cable (with two connectors!) between the power sink and the power source. Someone will accidentally pull a plug far more often than there will be a blackout. Of course, the blackout will affect all the machines... – mpez0 Feb 15 '10 at 19:22
  • well, if one cable is pulled, nothing bad will happen. and btw, my server room has two signs on the door: "DO NOT CLEAN IN HERE!!!" and "The sysadmin doesn't drink candy or flowers" – dyasny Feb 15 '10 at 21:49

Are you sure your UPS can handle to load of 4 additional servers?

Given the circumstances it would make the most sense to plug one PS into the UPS and the other PS to a surge-protected wall source. The servers can run with just power from the UPS to the one PS. The odds of a PS going bad at the same time as a power outage is very rare, though possible.


I agree with the previous answer. Also are all of you servers conencted to the ups correctly? For instance in a power outage, does the UPS software actually shutdown your machines gracefully. If not, they you are just as well off to plug the servers into your wall socket.


The moment the power goes out I predict that your UPS will run out of power before your servers can turn off.

In general, UPSes are used as a temporary stop-gap measure before a diesel generator can kick in.


It's been my experience that the UPS will fail far more often than the power supplies.

Also, the UPS is usually less reliable than the city power, and the power supplies are far more reliable than either.

That isn't to say that you shouldn't use a UPS, but in this situation, you really ought to have two UPSs, and make it so that a single UPS failure won't affect anything.

I've found that 2 power connections are most useful if you want to move a server from one outlet to another, or one part of the room to another, you can just walk it from outlet to outlet.

And, for goodness sake, monitor your UPSs. A UPS you don't pay attention to will interrupt service more often than anything else.


If you're plugging all your redundant PS onto the UPS, then you're wasting half of your connections. You could theoratically connect double the number of servers to the UPS and plug their redundant PS to a PDU. However, be very careful about the power draw and the runtime you're getting from your UPS with more servers attached to it.

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