Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION, Руководство по установке

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The Kernel
355
For instructions on using the boot loader to supply command line arguments to the kernel, refer to
Chapter 9, The GRUB Boot Loader
. For information on changing the runlevel at the boot loader
prompt, refer
Section 9.8, “Changing Runlevels at Boot Time”
.
The boot loader then places one or more appropriate
initramfs
images into memory. Next, the kernel
decompresses these images from memory to
/sysroot/
, a RAM-based virtual file system, via
cpio
.
The
initramfs
is used by the kernel to load drivers and modules necessary to boot the system. This
is particularly important if SCSI hard drives are present or if the systems use the ext3 file system.
Once the kernel and the
initramfs
image(s) are loaded into memory, the boot loader hands control
of the boot process to the kernel.
For a more detailed overview of the GRUB boot loader, refer to
Chapter 9, The GRUB Boot Loader
.
33.2.2.1. Boot Loaders for Other Architectures
Once the kernel loads and hands off the boot process to the
init
command, the same sequence of
events occurs on every architecture. So the main difference between each architecture's boot process
is in the application used to find and load the kernel.
For example, the Itanium architecture uses the ELILO boot loader, the IBM eServer pSeries
architecture uses yaboot, and the IBM System z systems use the z/IPL boot loader.
33.2.3. The Kernel
When the kernel is loaded, it immediately initializes and configures the computer's memory and
configures the various hardware attached to the system, including all processors, I/O subsystems,
and storage devices. It then looks for the compressed
initramfs
image(s) in a predetermined
location in memory, decompresses it directly to
/sysroot/
, and loads all necessary drivers. Next, it
initializes virtual devices related to the file system, such as LVM or software RAID, before completing
the
initramfs
processes and freeing up all the memory the disk image once occupied.
The kernel then creates a root device, mounts the root partition read-only, and frees any unused
memory.
At this point, the kernel is loaded into memory and operational. However, since there are no user
applications that allow meaningful input to the system, not much can be done with the system.
To set up the user environment, the kernel executes the
/sbin/init
program.
33.2.4. The
/sbin/init
Program
The
/sbin/init
program (also called
init
) coordinates the rest of the boot process and configures
the environment for the user.
When the
init
command starts, it becomes the parent or grandparent of all of the processes that
start up automatically on the system. First, it runs the
/etc/rc.d/rc.sysinit
script, which sets
the environment path, starts swap, checks the file systems, and executes all other steps required
for system initialization. For example, most systems use a clock, so
rc.sysinit
reads the
/etc/
sysconfig/clock
configuration file to initialize the hardware clock. Another example is if there are
special serial port processes which must be initialized,
rc.sysinit
executes the
/etc/rc.serial
file.