Relational databases (except maybe MemQL) treat the file system as the source of truth. And usually the file system is bigger then the memory so they need to co

Relational databases (except maybe MemQL) treat the file system as the source of truth. And usually the file system is bigger then the memory so they need to constantly update their cache with more relevant data. Redis loads everything to memory. And doesn't keep the structure in the file system, only the log, recreating it from log+snapshot.