Ceph
Distributed storage system that exposes object, block and file storage from the same cluster — Sage Weil's PhD project (2007) that became the open-source standard for on-prem storage.
The core is RADOS (Reliable Autonomic Distributed Object Store). Data placement uses the CRUSH algorithm — a deterministic function that lets clients compute where data lives without a metadata server. Three interfaces on top: RGW (S3/Swift), RBD (block for KVM/Kubernetes) and CephFS (POSIX filesystem).
Powers CERN, OVH, Bloomberg and most university clouds. Complex to operate — Rook turns it into a Kubernetes operator to tame the config jungle.