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I have to replace a very old UPS. We've got a new one. I'm not really a sys admin, but have to act the part, as the previous one left and, well there's no one else. He had it set up with 2 UPS's, and split the power supplies from each server (5 total) so that 1 power supply went to 1 UPS, and the other power supply went to the second UPS. We've got the new UPS in place. The former sys admin said that the second UPS would be enough to power the servers, while we put in the new UPS. I just want to double check; will this work?

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up vote 3 down vote accepted

Depends on the UPS. The fact that the servers have redundant power supplies means that they should be fine on just one power feed.

But that's assuming that the one power feed can provide the wattage that they need, and there's the rub -- dropping one UPS out will mean that the other UPS will have its load approximately doubled.

A UPS device is rated for a certain load, and for the decent ones you'll have a way to check what load it's under; an LCD display on the device, or a network connection with a reporting interface. In any case, they're usually rated in both volt-amps and watts - with server equipment, the watts metric is likely to be your limiting factor (since the power factor of computer power supplies is usually right near 100%).

If you have info on the load on the devices, check where they're at and what they're rated for. If not, I'd say schedule some maintenance and shut off everything that you can (ideally, more than half) before dropping to one power path. And be ready for a failure of the single UPS, even then.

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Good idea. I am certain I can shut down 1 server during this maintenance, and perhaps 2. – Rod Apr 18 '12 at 14:04

If you're unsure (and none of us can give you an informed answer without knowing the specifics of the servers, power supplies and UPS'es) then schedule a maintenence window, shut the servers down, and replace the old UPS.

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If you have to cut the mains power it will depend on the load the servers place on the UPS, how long it takes to install the new UPS (i.e. how long you need to run from the batteries) and if the power supplies failover correctly (i.e. draw power from the other UPS, which they most likely will).

Just be aware, as the batteries in your UPS age, they may provide less uptime than you expect.

As has been said before, schedule some maintenance time.

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I wonder if you have to disconnect old UPS completely, then replace it with new one (say, you'll need to install new in place of old), and connect only after that, or you can add new one, reconnect power supples to new one one-by-one, and then remove old UPS?

The first case is the worst one: when you disconnect old UPS, your first old UPS (the one that'll stays in place) will be overloaded (and don't forget it is old and batteries are wearied out) as soon as you disconnect second old UPS. So in case of power outage in that very moment, chances are it won't be able to supply your power needs and servers are finally be off.

The second case looks like no-risk (well, almost no risk, as doing anything with power systems without fully off anything), like 'poor man no risk'.

But if you ask me and you can afford that, I'd schedule a maintenence window, shut the servers down, and do everything with power systems and distribution the right way, without any fear for whatever can happen.

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Most or even all answers here are correct, but the only way to be sure is to calculate power consumption of servers and compare it to UPS rated load. It should take between 2 and 3 minutes to do so.

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Surely keep in mind that rated power is much higher than real-life consumption. If UPS are good, they are able to report real power use so have a look it it, then. – Alexander Apr 18 '12 at 11:22

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