Comment by huntaub

Well, I think this is what our company, Archil, is working on. We basically built an SSD clustering layer that proxies/caches/and assembles requests into object storage so that you can run a POSIX file system directly on top.

There's also some really great projects like SlateDB in this space, which could be more like what you're looking for (~RocksDB like API that runs on S3).