Mailing the story:

You probably know that reading from RAM is a lot of faster than reading files from the hard drive, and reduces your disk I/O. This article shows how you can store files and directories in memory instead of on the hard drive with the help of tmpfs (a file system for creating memory devices). This is ideal for file caches and other temporary data (such as PHP's session files if you are using session.save_handler = files) because the data is lost when you power down or reboot the system.