TMPFS is a virtual memory-based filesystem that can grow and shrink according to its content. Although its content is not saved across reboots, it is quite useful for storing temporary files. Hence, instead of mounting all the directories from a single filesystem, you can choose to mount directories that do not require permanent storage, such as /tmp, on TMPFS. Because content stored on TMPFS is not saved across reboots, however, essential directories such as /usr, /etc, or /bin cannot be stored on TMPFS. To use TMPFS, enable the "Virtual memory file system support (former shm fs)" item in the "File systems" submenu in the kernel configuration menu.
With kernel support for TMPFS enabled, you can mount a 4 MB TMPFS filesystem on /tmp, for example:
# mount -t tmpfs none /tmp -o size=4m
Alternatively, you can add a line in your /etc/fstab file and modify your /etc/init.d/rcS file to mount TMPFS at boot time. If you do not provide a size limit, the filesystem will grow according to its content.
In contrast with most other mount commands, TMPFS does not require a device or file to be mounted, hence the use of none as the device. The name of the device for TMPFS is actually ignored by mount, and replacing none by any other name would have no effect on the command.
If you would like more information regarding TMPFS, take a look at part three of the IBM developerWorks filesystem series mentioned earlier, Using the virtual memory (VM) filesystem and bind mounts.