The lithium-ion battery packs have other advantages, too. There's one to each half-rack of servers, so if a battery pack fails the problem will be isolated to that group of servers. By contrast, in some large data centers, a single UPS supports hundreds of racks of equipment.
"Most applications are good at handling small scale-failures, but almost no apps are good at handling a massive concurrent failure," Corddry says. "Big central UPSes can cause a massive concurrent failure. Once the UPS goes, you have no line of defense — you're down."
Putting the battery packs close to the servers is also more energy efficient, he says, because the power doesn't traverse as many conversion points as with an external UPS. And he expects the lithium-ion batteries to have a slightly longer life span, though time will tell.
Facebook has just started pilot testing the battery packs in its data centers, he said. It runs a "synthetic load" for a couple of weeks to make sure the technology will hold up, then switches to live customer data after that.
"Our plan is to have our mass-production gear hitting the data center next year," Corddry said.
He wouldn't quantify the exact cost savings over lead-acid batteries, but if Facebook is making the change throughout its infrastructure, it's safe to assume they could be significant.
If all goes well, there's no reason lithium-ion won't find its way into enterprise data centers. Facebook is designing the battery pack into the second version of the Open Compute Project's power shelf, which includes all the power components for a server rack. The first version works with the big lead-acid battery cabinets.
Like other OCP projects, the goal is to come up with a more energy-efficient and flexible design than those offered by the major manufacturers. Any company can pick up the design and build it.
There are six battery packs in the v2 power shelf, including two redundant ones in case of failure, for a total 13.2 kW of backup power. That's less than some high-performance computing centers need, but for Facebook — and most other businesses — it's plenty.
"People think of lithium-ion as a premium, expensive thing, but this is actually cheaper for us," Corddry said.
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