Many databases implement their own filesystem internally that are heavily optimized for database-y use cases and access patterns while missing standard POSIX an

Many databases implement their own filesystem internally that are heavily optimized for database-y use cases and access patterns while missing standard POSIX and other features a "real" filesystem would have. When this filesystem is installed on top of the OS filesystem, there is a cost due to duplication of effort, design impedance mismatch, limitations of the OS filesystem, etc. This is partially mitigated by turning the files in the OS filesystem into a giant block store to minimize interaction with the OS filesystem. Some database filesystems can be installed directly on raw block devices if you desire with no OS filesystem in the middle. This usually offers significant performance and efficiency gains since everything above the raw hardware is purpose-built for the requirements of optimal database performance.