Ceph. Although it's slightly upside down with it's block / objects semantics (and as such, you don't exactly get the namespaced append-only semantics of Colossu

Ceph. Although it's slightly upside down with it's block / objects semantics (and as such, you don't exactly get the namespaced append-only semantics of Colossus), and doesn't depend on an excellent separate BigTable-like and Chubby-like layer underneath, instead rolling their own.