How to discover linux pathes where most HDD writing happen?

how to determine where most data are written on hdd, i mean which pathes.
Currently im having RedHat CentOS 5.10 64bit and im thinking about SSD + HDD setup of a new server so i want to discover where are data writen on HDD, so i can use classic HDD there and on the rest i will use SSD for best reading performance.