Why does rsync forks itself? And why one such forked process is almost kinda idle (as seen in iotop)?
Solution 1:
rsync is a program designed to be a client and server. The server reads and the client writes. Imagine that instead of a single computer, you had computers over network, I'm sure it's lot more clear if you think that way.
Then there is the controller. As IO operations tend to come with certain amount of risk, an IO issue shouldn't cause total blocking or a crash. So, it creates a fork for each connection and sits in the background.