initramfs-tools

Ben Hutchings

What is the initramfs for?

• Huge variety of disk drivers, net drivers and filesystems that may be needed to mount root
• Generic kernel shouldn't have all these built-in
• Root might also need RAID, LVM, dm-crypt, which generally need to be configured by userland
• Kernel needs a way to load modules and run scripts but it can't read them from the root filesystem
• Solution: boot loader provides a bundle of files (initramfs image) to the kernel

How the kernel uses an initramfs

• Kernel always mounts an initial root filesystem using either ramfs or tmpfs
• If the boot loader provides an initramfs image, kernel unpacks it into this filesystem, then runs /init
• The initramfs init system is then responsible for mounting the real root filesystem and running the real init
  • Although it is not required to hand over at all - e.g. debian-installer

Overview

• Started in Ubuntu in 2005 and adopted in Debian later that year
• mkinitramfs builds an initramfs image using the included scripts, necessary kernel modules, udev, and utilities from klibc-utils or busybox
• Uses build-time and boot-time hook scripts for extensibility
  • Used by btrfs-tools, cryptsetup, lvm2, mdadm, etc.
• Supports dependencies within each set of scripts

The init system

• Mount devtmpfs, devpts, procfs, sysfs, tmpfs at /run
• Run init-top scripts - includes starting udev
• Load explicitly configured modules
• Run init-premount scripts
• Run local-top or nfs-top scripts - e.g. mdadm setup
• Run local-premount or nfs-premount scripts - e.g. btrfs scan, resume from disk
• Mount root filesystem

Questions?

Credits

• Linux 'Tux' logo © Larry Ewing, Simon Budig.
  • Modified by Ben to add Debian open-ND logo
• Debian open-ND logo © Software in the Public Interest, Inc.
• Xzibit promotional photo © Retna, used for purpose of parody