[cjensen]

The author has failed to account for command latency. If you write some bytes, there are a bunch of hardware buffering delays in getting bytes to disk including seek and rotational latency.

Async I/O avoids this. You can tell the I/O subsystem what you want to read next even while doing a write. The I/O is posted to the disk in modern systems, and the disk will begin seeking to the read site in parallel with informing the OS that the write has completed. Posting I/O even helps for SSDs to avoid the idle time on the SSD media between write done and read start.

[mtdewcmu]

I think there is some amount of write back caching in the kernel so that the application doesn't have to wait for each individual chunk to go to disk before it can submit the next chunk. I believe there's a sync on either file close or process termination, or some combination.