There is no such thing as folders in Linux, DOS, and Windows. The right term is directory.
The /opt directory is not really use at all and it is stored under the root "/" directory. Using quotas will not work very effectivly as a real device. Another way is to use parted or qtparted to split a partition, but it is very risky and it creates downtime for backing up the data and making the partition. Another way is to use a loop device which is not risky as splitting a partition and it is easy to do. A good howto to use loop devices as a physical hard drive is at http://www.mega-tokyo.com/osfaq2/ind...0Under%20Linux