NYTimes has a post on uptime.

99.999% Reliable? Don’t Hold Your Breath

By RANDALL STROSS

AT&T’s dial tone set the all-time standard for reliability. It was engineered so that 99.999 percent of the time, you could successfully make a phone call. Five 9s. That works out to being available all but 5.26 minutes a year.

The author was able to get Google.

As for moving to 99.999, well, that may never come. “We don’t believe Five 9s is attainable in a commercial service, if measured correctly,” says Urs Hölzle, senior vice president for operations at Google. The company’s goal for its major services is Four 9s.

Google’s search service almost reaches Five 9s every year, Mr. Hölzle says. By its very nature, it is relatively easy to provide uninterrupted availability for search. There are many redundant copies of Google’s indexes of the Web, and they are spread across many data centers. A Web search does not require constant updating of a user’s personal information in one place and then instantly creating identical copies at other data centers.

Amazon

One of those services, the Simple Storage Service, or S3, allows companies to store data on Amazon’s servers. “We talk of ‘durability’ of data — it’s designed for Eleven-9s durability,” says James Hamilton, a vice president for Amazon Web Services. That works out to a 0.000000001 percent chance of data being lost, at least theoretically.

And threw in a Netflix blog post.

One thing that Google and other companies offering Web services have learned to do is to keep software problems at their end out of the user’s view. John Ciancutti, vice president for personalization technology at Netflix, wrote on the company’s blog in December about lessons learned in moving its systems from its own infrastructure to that of Amazon Web Services. He said Netflix had adopted a “Rambo architecture”: each part of its system is designed to fight its way through on its own, tolerating failure from other systems upon which it normally depends.

“If our recommendations system is down, we degrade the quality of our responses to our customers, but we still respond,” Mr. Ciancutti said. “We’ll show popular titles instead of personalized picks. If our search system is intolerably slow, streaming should still work perfectly fine.”

Watch for availability to be marketed more.