Why do servers have 2 psu?

Question

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.

Solution 1

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.

Solution 2

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. ;)

Solution 3

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 replace the power source (remove or replace a UPS for service, replace the PDU, change mains, ...), one leg at a time.
• 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.

Comments

[Eric]

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?

[dortegaoh]

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).

[Zac67]

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.

[Timothy Frew]

@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