Comment by marcan_42

    #!/usr/bin/python
    import os, sys, time, datetime

    t = datetime.datetime.now().isoformat()

    print(t)
    for i in range(5):
        time.sleep(1)
        print(4 - i)

    fd = os.open(sys.argv[1], os.O_RDWR|os.O_CREAT)
    os.lseek(fd, 0, 0)
    os.write(fd, b"test: " + t.encode("ascii") + b"\n");
    os.fsync(fd)
    print("done!")
    time.sleep(100)

Run that on a Mac Mini. Do it a couple times. Remember the timestamp of the last one. Let it count down, then pull the plug within a few seconds after "done!" shows up. Boot up again. The file contents will have reverted to a prior point in time.

This isn't some hypothetical thing, this is a trivial test you can do. fsync() on macOS does not guarantee data is on stable storage. And this is actually well documented.

Then if you want to see the performance problem, make it a loop instead and use `fcntl.fcntl(fd, fcntl.F_FULLFSYNC, 1)`. You'll get 40-odd IOPS, but at least your data won't disappear after power loss.