Do you really need a database for this? On a unix system, you should be able to: CRUD users, CRUD files and directories, grant permissions to files or directories Is there a decade-old software that provides a UI or an API wrapper around these features for a "Google Drive" alternative? Maybe over the SAMBA protocol?
How would you implement things like version history or shareable URLs to files without a database? Another issue would be permissions: if I wanted to restrict access to a file to a subset of users, I’d have to make a group for that subset. Linux supports a maximum of 65536 groups, which could quickly be exhausted for a nontrivial number of users.
The file system is already a database.