Availability and durability are different. S3 has 11 9's of durability. This means you can expect to lose one out of 100,000,000,000 objects stored in S3, dur

Availability and durability are different. S3 has 11 9's of durability. This means you can expect to lose one out of 100,000,000,000 objects stored in S3, during an unspecified period of time. S3 achieves this by using "erasure codes", distributing chunks across hundreds of hard drives, and quickly replacing lost chunks. Hadoop is a popular storage system that can do erasure coding on your own servers. To make it more durable, you will need many reliable hard drives. But I didn't find any indication that Hadoop detects and replaces lost chunks automatically, so its durability is severely limited compared to S3. S3 has 4 9's of availability. This means you must expect S3 to be inaccessible 0.0001 of the time, or about an hour per year.