Servers with redundant power supplies

[Ian Blundell]

All of my servers have redundant power supplies that automatically failover.  I have always plugged both power cords into the UPS for that server.  I was talking to TrippLite about a problem I was having with one of their UPS units and they said that I should only plug one power cord into the UPS and the other into a wall outlet. The reasoning was that the 2 power supplies could be overloading the UPS which I don't really buy since I am still only powering the one server aren't I?  The only extra load would be the fan on the second power supply wouldn't it?

I have questions about this configuration:
1.  If the power fails on the UPS but the wall outlet is still good then wouldn't the shutdown software shutdown the server even though there is still good power from the wall outlet?
2.  The UPS normally adds filters and power surge protection,  the power supply direct from the wall outlet would bypass this.
3.  Even if I added a second UPS for the second power supply then would the shutdown software be smart enough to only shutdown the server as the second UPS was running out of power?
4.  Is there a way of knowing and/or controlling which of the power supplies is "primary"?
4.  I am also having problems getting the TrippLite software to run on server 2008 and recognize the UPS.  Do I even need the shutdown software to gracefully shutdown the server?  By the time the UPS is running out of juice then the workstations are all going to be off and there shouldn't be any activity on the server and would it be OK to let it just shutdown when the power runs out?

[Jeff Golas]

I'd leave em both on the UPS.

[Hans Manhave]

Questions I have not really considered.  I have two circuits, two UPS units and one redundent power supply per UPS/circuit.  I didn't install any auto shutdown system.  I have have 30-45 minutes of UPS time and like Ian says, all the work stations will be off by that time.  If I'm present, I will shut down the server if power is out still after 20 or 25 minutes which rarely happens, but does happen.

[Jeff Zylstra]

There's an easy way to find out.  Buy one of those watt miser type appliances that measure how many watts you are drawing, and then alternately plug your server's power supplies into each one to measure power drawn.  They're only about $35, so this kind of expenditure for a server is probably warranted.

[Charlie Charbonneau]

Forgive my ignorance, but isn't the point of redundancy to have the second power supply on an entirely different power source, say a back-up generator or something similar?  having it plugged into the same outlet/ups would really only protect against failure of the physical power supply itself. It seems like it'd be pointless to have a redundant power supply on the same ups.  I could see perhaps a secondary ups for that little extra time in a power outtage, but even that's questionable.

[Jeff Zylstra]

Forgive my ignorance, but isn't the point of redundancy to have the second power supply on an entirely different power source, say a back-up generator or something similar?  having it plugged into the same outlet/ups would really only protect against failure of the physical power supply itself. It seems like it'd be pointless to have a redundant power supply on the same ups.  I could see perhaps a secondary ups for that little extra time in a power outtage, but even that's questionable.

You're displaying wisdom, not ignorance.  I can say that since I'm of the same opinion.     Although servers should be on a dedicated circuit, I bet you that half of the folks here don't have it set up that way <sound of scurrying IT folks checking electrical connections>.   Electricians will tell you that this save a lot of hassles.  That said, if that circuit breaker trips, I would think that a failover should happen and the another live circuit and power supply should take over.

[Billy Welsh]

Forgive my ignorance, but isn't the point of redundancy to have the second power supply on an entirely different power source, say a back-up generator or something similar?  having it plugged into the same outlet/ups would really only protect against failure of the physical power supply itself. It seems like it'd be pointless to have a redundant power supply on the same ups.  I could see perhaps a secondary ups for that little extra time in a power outtage, but even that's questionable.

You're displaying wisdom, not ignorance.  I can say that since I'm of the same opinion.     Although servers should be on a dedicated circuit, I bet you that half of the folks here don't have it set up that way <sound of scurrying IT folks checking electrical connections>.   Electricians will tell you that this save a lot of hassles.  That said, if that circuit breaker trips, I would think that a failover should happen and the another live circuit and power supply should take over.

Also depends on the limitations of your space.  If your server closet was originally something else, and it got "converted" to the data closet, you may not have the "luxury" of 2 dedicated circuits.  At least you are getting protection from the main power supply in the server going out - I agree it is a minimum level of protection but it's still better than relying on a single power supply.  I have changed my share of those, some of them in servers, so I am very glad to know there are 2 in my current box.

[Jeff Zylstra]

Power supplies are one of the components most likely to die on a server.  Unfortunately, they're not "universal" anymore.  Especially in the Dell machines that I've worked on.  More and more are proprietary.  That's a shame because I still have a 600 Watt spare in my computer room that I'll probably never use.

[Hans Manhave]

One reason to put either both PS units on the same UPS or each PS on its own UPS could be to have the power filtered from surges and other nasty evil doers.  Without a UPS one exposes the heart of the operation to a higher likelihood of harmful electrical mishaps.  Sure, we have a breaker box protection device.  I have multiple circuits going into my closet.  And I'm fortunate enough with this 100+ degree weather to have three a/c units each capable of cooling my office/closet to acceptable temperatures.  But no power feed should be left unprotected from surges etc.

[Bloody Jack Kidd]

primarily, redundant power supplies are to protect you from PSU failures - not to power alternate power feeds from different inputs. However, they can be used that way to extend your N+1 practices. I would not plug one into a UPS and another into a wall socket though, that seems odd.

[Ben Thoele]

I had two APC SUA1500s that were setup as dual path power to two dedicated ground circuits and feeding our two Dell R610s.  We put a AP9618 management card in one of the two APCs since it a virtual environment.  Since the Sealed Lead Acid Batteries were end of life and they don't last long in a UPS, which is a little easy bake oven I externalized then.  I drilled a hole in the side of the case the ran the cable connector out to some large deep cycle solar grade batteries.  The only issue we had was the voltage range of a Flooded Lead Acid is a little slower than a Sealed Lead Acid Batt so the UPS doesn't even touch the capacity of the new batteries and the UPS cuts out sooner than we hoped.

[Mark]

Your UPS should be specked to support your specific server, including number of power supplies, and also along with anything else plugged into it.  UPS's are almost ALWAYS specked incorrectly.  We tend to keep them longer than other hardware, and they are expensive.  Hell, even Dell will sell you and underpowered UPS when you buy it as a package deal with new servers!

All that said, plug BOTH of them into the SAME UPS.  Sure, you could try to balance them between UPS's but I'd suggest shying away from that.  If you get an additional UPS, balance the load by device/power usage -- not by power supplies.  Besides the fact that you'd likely end up out of balance, you're not really doing yourself any favors by splitting them, in my opinion.  Been there, done that.

Also, I haven't installed any additional software to monitor UPSs.  I wanted to at some point and drive myself crazy trying to find something to fit what I pictured.  The end result for me was just the Windows built-in battery meter and some scripts.  The nice thing about using scripts is that you can use IF statements, you can remotely shutdown everything from a single source, you can send emails and text messages (or if you're using Asterisk, even trigger an automated phone call!).  I'd suspect you'd have to pay money to get all those features in the same product.

Just a reminder: these are my opinions.  Some directly from experience, some just based on experience + additional consideration.

[Jeff Zylstra]

I don't run any UPS software on any of my computers.  I bring in my laptop about once a year and plug it into each one of the UPS units to check remaining battery life, etc....  If it gets less than 10 minutes, I replace the battery.  UPS units are hard on batteries because they're constantly overcharging them.  Most batteries are designed to be discharged and then recharged.  If you don't do that, you can easily create a "memory" in the battery to the point where it won't hold as much of a charge anymore. 

[Mark]

I think that's old tech, Jeff.  what kind of UPS are you using?  My UPS tells me the load, the charge, and when to change the batteries.  See pic.

EDIT:  Also, my UPSs are under spec'd based on everything I said in previous post.  In addition to that, they are only meant to keep the systems alive long enough to shut down gracefully.  If we want an hour of battery life, we'd be looking at nearly $10,000 for a single UPS.   The cheapest one I spec'd on APC.com was around $7,700 I think.  We have a decent generator, so if the power is out long enough, I just fire that baby up, run some custom extension cords, and we're back in business.  Of course, I plug each UPS into the generator to ensure clean power.

[Mark]

Here is the direct link for the APC calculator: http://www.apc.com/tools/ups_selector/index.cfm

Choose the one in the middle for "Configure by devices"