fsync(2) is a somewhat expensive operation that flushes writes to the underlying disk/SSD. It's typically used by databases to ensure that writes survive very hard failure scenarios like your cat kicking the plug out of the wall. Synchronizing isn't automatically done by `flush`ing (from the `std::io::Write` or `tokio::io::AsyncWriteExt` traits). From the [`tokio::fs::File`] moduledocs: > To ensure that a file is closed immediately when it is dropped, you > should call `flush` before dropping it. Note that this does not ensure > that the file has been fully written to disk; the operating system > might keep the changes around in an in-memory buffer. See the > `sync_all` method for telling the OS to write the data to disk. [`tokio::fs::File`]: https://docs.rs/tokio/latest/tokio/fs/struct.File.html