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SvennD
Asked: 2015-04-17 08:39:39 +0800 CST

Double UPS on high load during power down

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To sketch the situation :

  • a bunch of servers (~15 going from 4u to 1u, some redudant PSU some single)
  • 2 UPS (1 APC I can read using apcupsd)
  • 2 PDU

So I came upon this situation, and I honestly have no idea what the best solution would be. We have currently put most of the servers (even redundant PSU) on 1 UPS (older one, no APC) Then when we started drawing too much juice from it, it started to complain and they bought another one, and changed the probably most power hungry servers (clusters) on the new UPS (APC)

I don't mind servers going offline during a power break, and I would like them to go down well before the UPS give out. Currently the servers run until they don't get any more power, then die painfully.

So the reasonable situation would be that we dedup the servers who can so that they pull 50/50 from each UPS (as far as possible). However, during a power down, one of the batteries would be finished before the other one, this would result that ALL the servers draw juice from the other UPS, probably pulling way to much power and resulting in a instant power break?

So my questions are do PSU pull 50% from each power source? And how can I (if necessary with new hardware) fix this situation. And what is the worst that can happen when you pull too much from a UPS ?

I want to protect the servers from high peaks in power and in short outages of <5 min. Anything beyond this time-frame is explainable up the ladder. Or servers are configured OK and we know they can power cycle if deemed necessary.

supermicro

2 Answers

  1. A few questions to answer here...

    As for the power supply distribution in dual PSU servers, the short answer is that it depends on the server manufacturer and system/BIOS configuration.

    Please see: How do servers with redundant power supplies balance consumption?

    With regard to using multiple UPS units, you should try to match the specs and load. I find it simpler to go with a single large UPS or multiple right-sized UPS units that serve logical groupings of equipment (e.g. Network rack, server rack).

    I'm not a fan of the half UPS/half utility power approach. That introduces too many unknowns when you start mixing in equipment with single PSUs and trying to calculate loads.

    Also see: Is there any value in protecting both rack power feeds with their own UPS?

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  2. Best Answer

    If you don't mind the downtime, just set the servers to power off as soon as (or very soon after) the power goes out. That way you won't deplete either battery.

    I personally would connect one PS of dualPS servers to the UPS (either of those) and the other one directly to power grid. That will simple up your load balancing.

    Best bet at loadbalancing would be to measure the power usage of each server at expected load and then dividing the load according to UPS specs.

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