How To Use Hard Disk As RAM In Ubuntu Linux

I heard that it is possible to use a portion of hard disk as RAM in Windows. I wondered if this trick is available for Ubuntu, so I googled about it and couldn’t find anything related to it. But I found out an AskUbuntu answer of using USB sticks as RAM. I did the same trick on Hard Disk and it works !! This trick can be accomplished with the use of some small commands in Terminal.