Data Placement in Swift

“One of the hard problems that needs to be solved in a distributed storage system is to figure out how to effectively place the data within the storage cluster. Swift has a “unique-as-possible” placement algorithm which ensures that the data is placed efficiently and with as much protection from hardware failure as possible.

Swift places data into distinct availability zones to ensure both high durability and high availability. An availability zone is a distinct set of physical hardware with unique failure mode isolation. In a large deployment, availability zones may be defined as unique facilities in a large data center campus. In a single-DC deployment, the availability zones may be unique rooms, separated by firewalls and powered with different utility providers. A multi-rack cluster may choose to define availability zones as a rack and everything behind a single top-of-rack switch. Swift allows a deployer to choose how to define availability zones based on the particular details of the available infrastructure…”

http://swiftstack.com/blog/2013/02/25/data-placement-in-swift/