> It's the same in this context. fsync uses a blocking API to implement atomicity No, it doesn’t. In that context fsync is a write barrier. It doesn’t make wr

> It's the same in this context. fsync uses a blocking API to implement atomicity No, it doesn’t. In that context fsync is a write barrier. It doesn’t make writes atomic. The difference is this: If you issue a handful of writes then your computer crashes, some, all or none of the writes may have completed. And some writes might be “skewed” - where part of the write succeeded and part did not. Atomicity means a set of actions happens as one indivisible unit, and it either all happens or all doesn’t happen. Think database transactions. Filesystem writes are not like that. They are not atomic operations[1]. Fsync does not change the behaviour of the write system call. Fsync only stalls your program until all writes have been written to disk, at least assuming your drive isn’t lying to you. Just like a memory barrier. If your computer crashes before fsync returns, all bets are off. [1] With the exception of aligned writes under the device block size. Allegedly. I’d still like to see this actually documented somewhere.