compare btw : RSYNC, DRBD, UNISON

src:
http://qq85609655.iteye.com/blog/1879416

mypdf:
https://drive.google.com/file/d/0B543p5wfySTQNmNyRVhjMFd6bHc/view?usp=sharing

notes:
- [TODO] need further check for features