> This means that plenty of algorithms are correct with unbuffered writers and are incorrect with buffered writers. I’ve been bitten by this and diagnosed bugs caused by this multiple times.

But write() on POSIX is also a buffered API. Until your program calls fsync / fdatasync, linux isn't required to actually flush anything to the underlying storage medium. And even then, many consumer storage devices will lie and return from fsync immediately before data has actually been flushed.

All the OSes that I know of will eagerly write data instead of waiting for fsync, but there's no guarantee the data will be persisted by the time your write() call returns. It usually isn't. If you're relying on write() to durably flush data to disk, you've probably got correctness / data corruption bugs lurking in your code that will show up if power goes out at the wrong time.