Do Ripple servers use RDBMS?
If so, which one?
I think I remember reading that Bitcoin uses some sort of file database. Same for Ripple?
The ripple server uses SQLite for structured data and a configurable "back end" for unstructured "bulk" storage.
The structured data consists of things like transactions indexed by which accounts they affected. The unstructured data consists of "chunks" of data indexed by hash that constitute portions of network history.
The preferred back end for bulk storage is currently RocksDB on Linux platforms.