A ring represents a mapping between the names of entities stored on disk and their physical locations. There are separate rings for accounts, containers, and objects. When other components need to perform any operation on an object, container, or account, they need to interact with the appropriate ring to determine their location in the cluster.
The ring maintains this mapping using zones, devices, partitions, and replicas. Each partition in the ring is replicated, by default, three times across the cluster, and partition locations are stored in the mapping maintained by the ring. The ring is also responsible for determining which devices are used for handoff in failure scenarios.
Data can be isolated into zones in the ring. Each partition replica is guaranteed to reside in a different zone. A zone could represent a drive, a server, a cabinet, a switch, or even a data center.
The partitions of the ring are equally divided among all of the devices in the Object Storage installation. When partitions need to be moved around (for example, if a device is added to the cluster), the ring ensures that a minimum number of partitions are moved at a time, and only one replica of a partition is moved at a time.
Weights can be used to balance the distribution of partitions on drives across the cluster. This can be useful, for example, when differently sized drives are used in a cluster.
The ring is used by the proxy server and several background processes (like replication).
These rings are externally managed, in that the server processes themselves do not modify the rings, they are instead given new rings modified by other tools.
The ring uses a configurable number of bits from a path’s MD5 hash as a partition index that designates a device. The number of bits kept from the hash is known as the partition power, and 2 to the partition power indicates the partition count. Partitioning the full MD5 hash ring allows other parts of the cluster to work in batches of items at once which ends up either more efficient or at least less complex than working with each item separately or the entire cluster all at once.
Another configurable value is the replica count, which indicates how many of the partition-device assignments make up a single ring. For a given partition number, each replica’s device will not be in the same zone as any other replica's device. Zones can be used to group devices based on physical locations, power separations, network separations, or any other attribute that would improve the availability of multiple replicas at the same time.
