In OpenStack Object Storage, data is placed across different tiers of failure domains. First, data is spread across regions, then zones, then servers, and finally across drives. Data is placed to get the highest failure domain isolation. If you deploy multiple regions, the Object Storage service places the data across the regions. Within a region, each replica of the data should be stored in unique zones, if possible. If there is only one zone, data should be placed on different servers. And if there is only one server, data should be placed on different drives.
Regions are widely separated installations with a high-latency or otherwise constrained network link between them. Zones are arbitrarily assigned, and it is up to the administrator of the Object Storage cluster to choose an isolation level and attempt to maintain the isolation level through appropriate zone assignment. For example, a zone may be defined as a rack with a single power source. Or a zone may be a DC room with a common utility provider. Servers are identified by a unique IP/port. Drives are locally attached storage volumes identified by mount point.
In small clusters (five nodes or fewer), everything is normally in a single zone. Larger Object Storage deployments may assign zone designations differently; for example, an entire cabinet or rack of servers may be designated as a single zone to maintain replica availability if the cabinet becomes unavailable (for example, due to failure of the top of rack switches or a dedicated circuit). In very large deployments, such as service provider level deployments, each zone might have an entirely autonomous switching and power infrastructure, so that even the loss of an electrical circuit or switching aggregator would result in the loss of a single replica at most.
