How the File system works when we start to write to a File

Start with the boundary between visibility and durability. An application writes through a file descriptor; the kernel usually copies bytes into the page cache and may return before they reach persistent media. The filesystem tracks metadata and block allocation, while a background writeback path eventually sends dirty pages to the device. fsync or an equivalent flush asks for stronger durability but adds latency and still needs a carefully designed update protocol when multiple files or records must remain consistent. Journaling protects selected metadata or data ordering depending on configuration; it is not a substitute for defining crash-safe application semantics. Discuss partial writes, append behavior, concurrent writers, permissions, full disks, and the difference between SSD device caches and durable media. Ask the interviewer what operating system and durability guarantee matter before asserting exact ordering.

Complexity: For a write of B bytes, the application-to-page-cache work is O(B); metadata lookup is typically O(log n) in an indexed directory with n entries, while device latency is workload-dependent.