Novell LINUX ENTERPRISE SERVER 10 - INSTALLATION AND ADMINISTRATION 11-05-2007, Installation Manual

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disk are referred to as the Master Boot Record (MBR). The boot loader then
passes control to the actual operating system, in this case, the Linux kernel. More
information about GRUB, the Linux boot loader, can be found in
Chapter 20, The
Boot Loader (page 387). For a network boot, the BIOS acts as the boot loader. It
gets the image to start from the boot server then starts the system. This is completely
independent of local hard disks.
3. Kernel and initramfs To pass system control, the boot loader loads both the
kernel and an initial RAM–based file system (initramfs) into memory. The contents
of the initramfs can be used by the kernel directly. initramfs contains a small exe-
cutable called init that handles the mounting of the real root file system. If special
hardware drivers are needed before the mass storage can be accessed, they must
be in initramfs. For more information about initramfs, refer to
Section 19.1.1,
“initramfs” (page 372). If the system does not have a local hard disk, initramfs must
provide the root file system to the kernel. This can be done with the help of a net-
work block device like iSCSI or SAN, but it is also possible to use NFS as the root
device.
4. init on initramfs This program performs all actions needed to mount the
proper root file system, like providing kernel functionality for the needed file
system and device drivers for mass storage controllers with udev. After the root
file system has been found, it is checked for errors and mounted. If this has been
successful, the initramfs is cleaned and the init program on the root file system is
executed. For more information about init, refer to
Section 19.1.2, “init on
initramfs” (page 373). Find more information about udev in Chapter 24, Dynamic
Kernel Device Management with udev (page 461).
5. init init handles the actual booting of the system through several different levels
providing different functionality. init is described in Section 19.2, “The init Process”
(page 375).
19.1.1 initramfs
initramfs is a small cpio archive that the kernel can load to a RAM disk. It provides a
minimal Linux environment that enables the execution of programs before the actual
root file system is mounted. This minimal Linux environment is loaded into memory
by BIOS routines and does not have specific hardware requirements other than sufficient
memory. initramfs must always provide an executable named init that should execute
the actual init program on the root file system for the boot process to proceed.
372 Installation and Administration