Object Storage allows configuring zones in order to isolate failure boundaries. Each data replica resides in a separate zone, if possible. At the smallest level, a zone could be a single drive or a grouping of a few drives. If there were five object storage servers, then each server would represent its own zone. Larger deployments would have an entire rack (or multiple racks) of object servers, each representing a zone. The goal of zones is to allow the cluster to tolerate significant outages of storage servers without losing all replicas of the data.
As mentioned earlier, everything in Object Storage is stored, by default, three times. Swift will place each replica "as-uniquely-as-possible" to ensure both high availability and high durability. This means that when chosing a replica location, Object Storage chooses a server in an unused zone before an unused server in a zone that already has a replica of the data.
When a disk fails, replica data is automatically distributed to the other zones to ensure there are three copies of the data.
