UPSes in Datacentres

Posted: Tue, 23 August 2011 | permalink | 3 Comments

(This was going to be a comment on this blog post, but it’s a Turdpress site that wants JS and cookies to comment. Bugger that for a game of skittles.

Rimuhosting’s recent extended outage due to power problems was apparently caused by a transfer switch failure at their colo provider. This has led people to wonder if putting UPSes in individual racks is a wise move. The theory is that in the event of a small outage, the UPS can keep things humming, and in an extended outage you can gracefully shut things down rather than having a hard thump.

I happen to think this theory is bunkum. Your UPS is a newly instituted single point of failure. I’d be willing to bet that the cost of purchasing, installing, and maintaining the UPSes, as well as the cost of the outages that inevitably result from their occasional failure, would be far greater than the cost of the occasional power outage you get in a well-managed facility.

Good facilities don’t have small outages. They don’t have squirrels in the roof cavities, and they don’t have people dropping spanners across busbars. The only outages they have are the big ones, when some piece of overengineered equipment turns out to be not so overengineered – the multi-hour (or multi-day) ones where your UPS isn’t going to stop you from going down. Your SLA credit and customer goodwill is already toast, so all you’re saving is the incremental cost of a little bit more downtime while you get fscks run.

If you want the best possible power reliability, get yourself into a really well engineered facility, and run dual-power on everything. Definitely run the numbers before you go down the UPS road; I’ll bet you find they’re not worth it.


From: Warren
2011-08-23 11:35

Great, a UPS can keep your server alive.

Will a UPS keep the web of switches required to provide you a network connection?

What happens when they have an electrical fire and hit the big red Emergency Power Off button - and your rack doesn’t go down? Then your rack fries the firemen who use water on the datacenter. Good luck explaining that liability to the President of your company and Legal.

From: Matt Palmer
2011-08-23 11:50

Good points, Warren – the network issue isn’t a big one for the sort of use-case that was originally suggested (the idea was the UPS was merely a way to get an orderly shutdown) but the issues relating to problems “in the event of an emergency” are huge, and something that anyone would do well to bear in mind.

From: John Ferlito
2011-08-23 13:00

Also don’t forget the cost and hassle of regularly having to change batteries.

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