The Kernel Newbie Corner: “initrd” and “initramfs”–What’s Up With That?

“So… What Are They?

“If we can be a little sloppy for a minute or two, both of those
concepts refer to a simple idea — that of an “early userspace”
root filesystem that can be used to get at least the minimum
functionality loaded in order to let the boot process continue.
There’s a lengthy explanation in the kernel source tree in the file
Documentation/filesystems/ramfs-rootfs-initramfs.txt, but I’ll try
to simplify that just a bit.

“In a nutshell, when your bootloader (GRUB?) loads your Linux
kernel, it is of course the kernel’s job to finish the boot
process. But to do so, it might require particular drivers to be
able to work with, say, hardware RAID controllers, or a network,
and so on. And depending on where those critically important
drivers are, the kernel might not have the ability to load them;
hence, the creation of a preliminary root file system that would
contain just enough in the way of loadable modules to give the
kernel access to the rest of the hardware.

“Quite simply, it’s the bootloader’s job to pass control to the
kernel, hand it the “initrd” (initial ram disk), let the kernel
mount it and get what it needs, whereupon the kernel can toss the
initrd and replace it with the real root filesystem. With me so
far?”

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