Strong reliability and fast recovery — All data in Ceph is replicated across multiple OSDs. If any OSD fails, data is automatically re-replicated to other devices. However, unlike typical RAID systems, the replicas for data on each disk are spread out among a large number of other disks, and when a disk fails, the replacement replicas are also distributed across many disks. This allows recovery to proceed in parallel (with dozens of disks copying to dozens of other disks), removing the need for explicit “spare” disks (which are effectively wasted until they are needed) and preventing a single disk from becoming a “RAID rebuild” bottleneck.