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I have a Dell power edge 2900 server and it has 2 psu, so Can I run the server by having one psu plugged in. i am not going to have this server on all day and night. i am just using it to learn more about servers and networking. Trying to get my Cisco certs and network admin degree.

  • Yes you can run it with just one, because only one is used at one time. There are two for redundancy, meaning one as a backup in case the other one fails. If there are two issue your server already, no point in taking one out if they both work. – EarthMind Apr 15 '18 at 8:04
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    Multiple PSUs usually run load balanced ie. each PSU provides half the power. – Zac67 Apr 15 '18 at 9:06
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You have two power supplies for redundancy. Assuming they are hot-plug, if one fails you can replace it while the system is still running.

You could also run to separate servers in your data center and connect each power supply to a separate circuit. So if a something breaks one circuit your server would still be running.

  • Oh OK so it's kinda like another backdoor. Another question, So I can just have one psu connected and leave the other one unplugged? – Eric Apr 15 '18 at 8:19
  • Yes, usually you can run a server on only one PSU. Most servers however will complain in some way or another if not both are connected (high fan speed, beeping, or just showing a failure display). – Gerald Schneider Apr 15 '18 at 8:22
  • @GeraldSchneider That’s not true. Servers prefer two PSU’s if they support it. They can sometimes be configured to run with a balanced load for equal wear or in pure failover mode – Timothy Frew Apr 15 '18 at 10:39
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As two power supplies are for redundancy, a single PSU should always have enough capacity for the whole server: you could leave the other one unplugged, if you wish.

Redundancy for environmental problems comes usually prior to redundancy for PSU failures, as they occur more often, and a failing PSU may also harm other components. The units could have separate power sources altogether, or separated UPS solutions, so that a failing battery on a single UPS wouldn't cause power outages.

And for anyone who even thinks about it: they are NOT for moving a running server to another place. ;)

  • Yet a dual PSU enables you to change to power source on the fly: you can remove or replace a UPS without interrupting server power. – Zac67 Apr 15 '18 at 9:08
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For a more complete answer than the ones already present:

Two (or N+1) PSUs are about redundacy but not only that of the PSU - the other causes are much more likely. Multiple PSUs are usually also connected to multiple or redundant UPSes and power phases.

  • When a PSU fails, the other(s) can still supply power.
  • When a UPS fails power isn't interrupted.
  • You can remove or replace a UPS for service or when it has failed.
  • You can replace the PDU.
  • You can provide redundant power with just a single UPS - one PSU connects to the UPS, the other directly to mains (in case of UPS failure). Note that the UPS needs to keep enough headroom to support full server load.
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As others have stated you have multiple power supply units for redundancy. Normally in a data center you would connect one half of the power supply units to feed A and the other half to feed B. Than when the DC does maintenance on their feeds your equipment wont go down, crashing applications, corrupting a DB or something else. Or if a power supply unit physically fail your equipment wont go down either.

For however many PSU's you have. 2, 4, 6 or more you normally only need to connect half for normal server operation unless else is specified.

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