I’ve used this technique in the past, and the problem is that the way some file systems perform the file‑offset‑to‑disk‑location mapping is not scalable. It might always be fine with 512 MB files, but I worked with large files and millions of extents, and it ran into issues, including out‑of‑memory errors on Linux with XFS.

The XFS issue has since been fixed (though you often have no control over which Linux version your program runs on), but in general I’d say it’s better to do such mapping in user space. In this case, there is a RocksDB present anyway, so this would come at no performance cost.