SAN FRANCISCO--The inner workings of Google just became a little less secret.

The search colossus has shed only occasional light on its data center operations, but on Wednesday, Google fellow Jeff Dean turned a spotlight on some parts of the operation. Speaking to an overflowing crowd at the Google I/O conference here on Wednesday, Dean managed simultaneously to demystify Google a little while also showing just how exotic the company's infrastructure really is. On the one hand, Google uses more-or-less ordinary servers. Processors, hard drives, memory--you know the drill.

On the other hand, Dean seemingly thinks clusters of 1,800 servers are pretty routine, if not exactly ho-hum. And the software company runs on top of that hardware, enabling a sub-half-second response to an ordinary Google search query that involves 700 to 1,000 servers, is another matter altogether.

Google doesn't reveal exactly how many servers it has, but I'd estimate it's easily in the hundreds of thousands. It puts 40 servers in each rack, Dean said, and by one reckoning, Google has 36 data centers across the globe. With 150 racks per data center, that would mean Google has more than 200,000 servers, and I'd guess it's far beyond that and growing every day.

Regardless of the true numbers, it's fascinating what Google has accomplished, in part by largely ignoring much of the conventional computing industry. Where even massive data centers such as the New York Stock Exchange or airline reservation systems use a lot of mainstream servers and software, Google largely builds its own technology.

I'm sure a number of server companies are sour about it, but Google clearly believes its technological destiny is best left in its own hands. Co-founder Larry Page encourages a "healthy disrespect for the impossible" at Google, according to Marissa Mayer, vice president of search products and user experience, in a speech Thursday.

To operate on Google's scale requires the company to treat each machine as expendable. Server makers pride themselves on their high-end machines' ability to withstand failures, but Google prefers to invest its money in fault-tolerant software.

"Our view is it's better to have twice as much hardware that's not as reliable than half as much that's more reliable," Dean said. "You have to provide reliability on a software level. If you're running 10,000 machines, something is going to die every day."

Breaking in is hard to do
Bringing a new cluster online shows just how fallible hardware is, Dean said.

In each cluster's first year, it's typical that 1,000 individual machine failures will occur; thousands of hard drive failures will occur; one power distribution unit will fail, bringing down 500 to 1,000 machines for about 6 hours; 20 racks will fail, each time causing 40 to 80 machines to vanish from the network; 5 racks will "go wonky," with half their network packets missing in action; and the cluster will have to be rewired once, affecting 5 percent of the machines at any given moment over a 2-day span, Dean said. And there's about a 50 percent chance that the cluster will overheat, taking down most of the servers in less than 5 minutes and taking 1 to 2 days to recover.

A look at a custom-made Google rack with 40 servers from a modern data center. Infrastructure guru Jeff Dean showed the snapshot at the Google I/O conference.

While Google uses ordinary hardware components for its servers, it doesn't use conventional packaging. Google required Intel to create custom circuit boards. And, Dean said, the company currently puts a case around each 40-server rack, an in-house design, rather than using the conventional case around each server.

The company has a small number of server configurations, some with a lot of hard drives and some with few, Dean said. And there are some differences at the larger scale, too: "We have heterogeneity across different data centers but not within data centers," he said.

As to the servers themselves, Google likes multicore chips, those with many processing engines on each slice of silicon. Many software companies, accustomed to better performance from ever-faster chip clock speeds, are struggling to adapt to the multicore approach, but it suits Google just fine. The company already had to adapt its technology to an architecture that spanned thousands of computers, so they already have made the jump to parallelism.

"We really, really like multicore machines," Dean said. "To us, multicore machines look like lots of little machines with really good interconnects. They're relatively easy for us to use."

Although Google requires a fast response for search and other services, its parallelism can produce that even if a single sequence of instructions, called a thread, is relatively slow. That's music to the ears of processor designers focusing on multicore and multithreaded models.

"Single-thread performance doesn't matter to us really at all," Dean said. "We have lots of parallelizable problems."

The secret sauce
So how does Google get around all these earthly hardware concerns? With software--and this is where you might think about dusting off your computer science degree.

Dean described three core elements of Google's software: GFS, the Google File System, BigTable, and the MapReduce algorithm. And although Google helps with a lot of open-source software projects that helped the company get its start, these packages remain proprietary except in general terms.

GFS, at the lowest level of the three, stores data across many servers and runs on almost all machines, Dean said. Some incarnations of GFS are file systems "many petabytes in size"--a petabyte being a million gigabytes. There are more than 200 clusters running GFS, and many of these clusters consist of thousands of machines.

GFS stores each chunk of data, typically 64MB in size, on at least three machines called chunkservers; master servers are responsible for backing up data to a new area if a chunkserver failure occurs. "Machine failures are handled entirely by the GFS system, at least at the storage level," Dean said.

To provide some structure to all that data, Google uses BigTable. Commercial databases from companies such as Oracle and IBM don't cut the mustard here. For one thing, they don't operate the scale Google demands, and if they did, they'd be too expensive, Dean said.

BigTable, which Google began designing in 2004, is used in more than 70 Google projects, including Google Maps, Google Earth, Blogger, Google Print, Orkut, and the core search index. The largest BigTable instance manages about 6 petabytes of data spread across thousands of machines, Dean said.

MapReduce, the first version of which Google wrote in 2003, gives the company a way to actually make something useful of its data. For example, MapReduce can find how many times a particular word appears in Google's search index; a list of the Web pages on which a word appears; and the list of all Web sites that link to a particular Web site.

With MapReduce, Google can build an index that shows which Web pages all have the terms "new," "york," and "restaurants"--relatively quickly. "You need to be able to run across thousands of machines in order for it to complete in a reasonable amount of time," Dean said.

The MapReduce software is increasing use within Google. It ran 29,000 jobs in August 2004 and 2.2 million in September 2007. Over that period, the average time to complete a job has dropped from 634 seconds to 395 seconds, while the output of MapReduce tasks has risen from 193 terabytes to 14,018 terabytes, Dean said.

On any given day, Google runs about 100,000 MapReduce jobs; each occupies about 400 servers and takes about 5 to 10 minutes to finish, Dean said.

That's a basis for some interesting math. Assuming the servers do nothing but MapReduce, that each server works on only one job at a time, and that they work around the clock, that means MapReduce occupies about 139,000 servers if the jobs take 5 minutes each. For 7.5-minute jobs, the number increases to 208,000 servers; if the jobs take 10 minutes, it's 278,000 servers.

My calculations could be off base, but even qualitatively, that's enough computing horsepower to make the mind boggle.

Fault-tolerant software
MapReduce, like GFS, is explicitly designed to sidestep server problems.

"When a machine fails, the master knows what task that machine was assigned and will direct the other machines to take up the map task," Dean said. "You can end up losing 100 map tasks, but can have 100 machines pick up those tasks."

The MapReduce reliability was severely tested once during a maintenance operation on one cluster with 1,800 servers. Workers unplugged groups of 80 machines at a time, during which the other 1,720 machines would pick up the slack. "It ran a little slowly, but it all completed," Dean said.

And in a 2004 presentation, Dean said, one system withstood a failure of 1,600 servers in a 1,800-unit cluster.

Next-generation data center to-do list
So all is going swimmingly at Google, right? Perhaps, but the company isn't satisfied and has a long to-do list.

Most companies are trying to figure out how to move jobs gracefully from one server to another, but Google is a few orders of magnitude above that challenge. It wants to be able to move jobs from one data center to another--automatically, at that.

"We want our next-generation infrastructure to be a system that runs across a large fraction of our machines rather than separate instances," Dean said.

Right now some massive file systems have different names--GFS/Oregon and GFS/Atlanta, for example--but they're meant to be copies of each other. "We want a single namespace," he said.

These are tough challenges indeed considering Google's scale. No doubt many smaller companies look enviously upon them.

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