Early on, Linux supported initrd, which was just a ramdisk with a FS image written to it and mounted. Later, initramfs came along, which is all anyone uses any

Early on, Linux supported initrd, which was just a ramdisk with a FS image written to it and mounted. Later, initramfs came along, which is all anyone uses anymore. It is a cpio archive that is extracted onto a special form of tmpfs. One neat trick is that you can append multiple cpio archives, and the kernel will expand them one after another, so you can easily customize your distro-provided initramfs. Only the first one can be compressed, and you have to be careful about it getting overwritten by a kernel update, but it is handy for quick experiments. There is also pivot_root and switch_root, which have man pages if you're interested in the gory details. For all of initrd/initramfs's history, you've only needed one if your monolithic kernel couldn't mount the root filesystem. So if you needed a module, or had some special config that needed to happen that the kernel couldn't do internally.