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

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

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

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.

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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