以下摘自Specfs, Devfs, Tmpfs, and Others：
specfs – specfs, or Special FileSystem, is a virtual filesystem used to access special device files. This filesystem is odd compared to other filesystems in general because this filesystem does not require a mount-point, yet the OS can still use specfs. However, specfs can be mounted by the user (mount -t specfs none /dev/streams). The device files for character devices in the /dev/ directory use specfs.
devfs – devfs is a device manager in the form of a filesystem. The Device FileSystem is largely the same as specfs except for some differences in the way they function and their uses. devfs is used for most of the device files in /dev/. Most Unix and Unix-like systems use devfs including Mac OS X, *BSD, and Solaris. Nearly all Unix and Unix-like systems that use devfs place it on the kernelspace. However, Linux uses a userspace-kernelspace hybrid approach. This means the devfs virtual filesystem is on the kernelspace and userspace.
tmpfs – The Temporary filesystem is a virtual filesystem for storing temporary files. This filesystem is really in the memory and/or in the swap space. Obviously, all data on this filesystem are lost when the system is shutdown. The mount point is /tmp/.
devtmpfs – This is an improved devfs. The purpose of devtmpfs is to boost boot-time. devtmpfs is more like tmpfs than devfs. The mount-point is /dev/. devtmpfs only creates device files for currently available hardware on the local system.
总结一下：
devfs是文件系统形式的device manager。tmpfs存在在内存和swap中，因此只能保存临时文件。devtmpfs是改进的devfs，也是存在内存中，挂载点是/dev/。