Amazon S3 reliability

According to the Amazon Web Services Blog, they currently have 762 billion objects in Amazon S3. That's impressive. The popularity of Amazon S3 isn't hard to understand: it's easy to use, only $0.14 per gigabyte-month in most regions, and is “designed to provide 99.999999999% durability and 99.99% availability of objects over a given year” (quoted from Amazon Simple Storage Service (Amazon S3)).

That's an impressive statement: 99.999999999% durability. That means that for my 7,038,080 objects, I could expect to lose one in 14,208 years, or to put it another way I have a 0.007% chance of losing an object in a particular year. That seems like a pretty minuscule risk.

But then you look at the scale of Amazon Web Services. There are 762 billion objects in Amazon S3. That means by their design criteria (ignoring the reduced redundancy storage option) they expect to lose at least seven of those objects this year. Have you checked your objects today?

Now I doubt that the “99.999999999%” probability is a “normal operating conditions” number. I suspect that's a guess at the probability of three Amazon data centres being taken out at once, or something like that. In normal operations I suspect that you might as well just call it 100% reliable in terms of preserving your object. But I find it amazing that they're at the scale where such minuscule probabilities become certainties (if you naïvely apply them).