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In equipment which has dual power supplies - e.g. a server - how is power draw balanced between them?

Will they be "active/active", with load evenly spread between both PSUs? Or will they be "active/passive", with one PSU supplying most (all?) power required by the server, and the other drawing no power (or close to no power) but standing by ready to take over if required? Or something else?

For example:

We have a server with dual power supplies. Each power supply is on and connected to mains power. The server currently requires 400 watts of power.

Will 400W be supplied by PSU #1 (and the mains connection it uses), and 0W supplied by PSU #2?

-or-

Will 200W be supplied by PSU #1, and 200W supplied by PSU #2?

-or-

Something else?

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1 Answer

up vote 2 down vote accepted

I've seen it go both ways, actually. In some ancient HP ML530's that had 3 PSUs, load was balanced across all three. I believe the Dell servers of the era also did the same. Some of our newer stuff does either/or for load. I have a tape library with 2 PSUs that's either/or, and a shelf of disks that load-balances. Unfortunately, you just have to measure. If you're curious a Kill-o-Watt is handy for figuring this kind of stuff out.

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+1 exactly, it depends on the make and model of server. –  Chopper3 Oct 19 '10 at 5:54
    
Thanks for the info guys, appreciate it –  Matt Oct 19 '10 at 21:03
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