GETTING STARTED
WITH
SYSTEM BOOT PROCESS
LINUX ADMIN
Mr. RAM
SYSTEM BOOT PROCESS:
An important and powerful aspect of Red Hat Enterprise Linux is the open,
user-configurable method it uses for starting the operating system. Users are
free to configure many aspects of the boot process, including specifying the
programs launched at boot-time.
Below are the basic stages of the boot process for an x86 system:
The system BIOS / UEFI checks the system and launches the first stage
boot loader on the MBR / GUID Partition Table (GPT) of the primary
hard disk.
The first stage boot loader loads itself into memory and launches the
second stage boot loader from the /boot/ partition.
The second stage boot loader loads the kernel into memory, which in turn
loads any necessary modules and mounts the root partition read-only.
The kernel transfers control of the boot process to the /sbin/system or
init program.
The /sbin/systemd program loads all services and user-space tools, and
mounts all partitions listed in /etc/fstab.
The user is presented with a login screen for the freshly booted Linux
system.
UNDERSTANDING BOOT PROCESS IN DEPTH:
BIOS / UEFI:
BIOS and UEFI are two essential firmware interfaces responsible for
initializing hardware components, running system diagnostics, and
supporting the startup of the operating system on a computer.
BIOS (BASIC INPUT OUTPUT SYSTEM):
As soon as the computer is switched on, the BIOS assumes command and
executes the Power-On Self-Test (POST) to ensure the essential
hardware components – including the RAM, CPU, and storage devices –
are all functioning properly.
Once the boot device has been identified, the BIOS proceeds to search
for Master Boot Record (MBR) on the storage device. These contain
the crucial initial boot loader code.
LINUX ADMIN
Mr. RAM
MBR (Master Boot Record):
The MBR is located in the first sector of a storage device (usually the
first 512 bytes of a hard drive or SSD). It's in a fixed location, the
LBA (Logical Block Address) 0.
The Master Boot Record (MBR) if of 512 bytes in size. It consists of
three components:
The primary boot loader information occupies the initial 446 bytes.
Following that, the partition table information fills the subsequent
64 bytes.
Lastly, the MBR validation check, also known as the magic
number, resides in the final 2 bytes.
MBR has limitations in supporting only four primary partitions or
three primary partitions and one extended partition, which can further
contain multiple logical partitions. This restricts the number of
partitions usable on a disk.
MBR uses 32-bit addressing, limiting disk sizes to 2 terabytes (TB).
Larger disks cannot be fully utilized under MBR due to this limitation.
The BIOS then dutifully passes the reins to the designated boot loader,
such as GRUB / GRUB2 for Linux operating systems.
UEFI (UNIFIED EXTENSIBLE FIRMWARE INTERFACE):
UEFI is a more modern and versatile replacement for BIOS. It provides
more advanced features and capabilities than BIOS.
Similar to BIOS, UEFI starts with the hardware initialization and system
checks. But UEFI supports more modern hardware standards and allows
for faster boot times compared to traditional BIOS.
UEFI introduced Secure Boot, a security feature that verifies the digital
signatures of boot loaders and operating system kernels during the boot
process. This helps prevent the loading of unauthorized or malicious code
during boot time.
Once the boot device has been identified, the UEFI proceeds to search for
GUID Partition Table (GPT) on the storage device.
UEFI requires a specific partition known as the UEFI System Partition
(ESP), which is a primary component of the GPT scheme.
LINUX ADMIN
Mr. RAM
UEFI firmware uses information stored in the GPT to locate the UEFI
boot loader. The boot loader is stored in the ESP and is specified in the
firmware's boot configuration data.
GPT and UEFI work together to support Secure Boot functionality.
GPT supports disk sizes larger than 2TB, addressing the limitations of the
MBR partitioning scheme.
It efficiently manages partitions on larger disks and provides scalability
for future storage needs.
DIFFERENCES BETWEEN BIOS AND UEFI:
BIOS uses the Master Boot Record (MBR) method, while UEFI uses the
GUID Partition Table (GPT) method.
UEFI is more flexible and supports larger storage capacities, modern
hardware, and faster boot times compared to BIOS.
UEFI introduces Secure Boot, enhancing system security by verifying the
authenticity of bootloader and OS components.
GRUB / GRUB2:
GRUB stands for Grand Unified Boot Loader. It's an old boot loader in
the Linux world, responsible for managing the boot process of a computer.
RHEL7 is distributed with v2 of the GNU Grand Unified Bootloader
(GRUB 2), which allows the user to select an operating system or kernel to
be loaded at system boot time. GRUB 2 also allows the user to pass
arguments to the kernel.
GRUB 2 reads its configuration from the /boot/grub2/grub.cfg file on
traditional BIOS-based machines and from the
/boot/efi/EFI/redhat/grub.cfg file on UEFI machines.
INITRD (INITIAL RAM DISK) IMAGE:
The initrd (initial RAM disk) and initramfs (initial RAM File System)
are different methods we can use to load a temporary root file system to
the RAM for successful booting.
Initramfs/initrd is used as the first root filesystem that your machine has
access to.
LINUX ADMIN
Mr. RAM
Traditionally, initrd was used, but modern systems often use initramfs (a
more flexible successor). Initramfs is a cpio archive that is uncompressed
into a RAM disk at boot time.
It's more versatile, allowing for a more modular approach to including
essential files and drivers.
KERNEL:
The kernel is the core of the operating system, managing hardware
resources, providing abstractions, and controlling interactions between
hardware and software.
After the initrd image completes its tasks, the kernel takes control. It
initializes the system hardware, mounts the root file system, and begins the
user-space initialization process.
The kernel uses information provided by the initrd to mount the actual root
file system (for example, ext4, XFS) specified in the boot parameters.
RootFS:
The Root File System (rootfs) is a critical component in the booting
process of an operating system. It is the top-level directory hierarchy of
the file system and contains essential system files and directories.
In the context of the booting process, the root file system is the initial file
system that the operating system kernel mounts during the boot sequence.
INIT / SYSTEMD PROCESS:
It is the first process that runs in the system with process ID of 1.
After the kernel has loaded and initialized, it hands over control to the
system/init process.
Traditional Unix systems used the init process with different runlevels,
where each runlevel represented a different system state. But modern Linux
systems, including those based on Red Hat Enterprise Linux (RHEL), have
transitioned to using systemd, which serves as a replacement for init and
introduces a more flexible and efficient approach to managing system
initialization.
LINUX ADMIN
Mr. RAM
systemd reads its configuration from unit files located in directories such as
/etc/systemd/system and /usr/lib/systemd/system.
System daemons, also known as background processes or services
During the boot process, the init or systemd process is responsible for
starting system daemons.
Following examples of various systemd unit types:
Service : Controls and manages individual system services.
Target : Represents a group of units that define system states.
Device : Manages hardware devices and their availability.
Mount : Handles file system mounting.
Timer : Schedules tasks to run at specific intervals.
RUNLEVELS / TARGETS:
When a system starts, systemd activates the default.target symbolic link,
which points to the true target unit.
Following are the target groups:
poweroff.target (runlevel 0) : Poweroff or Shutdown the system.
rescue.target (runlevel 1) : launches a rescue shell session.
multi-user.target (runlevel 2,3,4): Non-graphical / multi-user system
graphical.target (runlevel 5) : Graphical multi-user interface
reboot.target (runlevel 6) : Reboots the system.
Once system daemons are initialized, the system is ready to handle user
interactions. That means the user is presented with a login screen for the
freshly booted Linux system.
LINUX ADMIN
Mr. RAM
CHANGING THE DEFAULT TARGET TO BOOT INTO:
Each target represents a certain level of functionality and is used for
grouping other units.
When you set a default target unit, the current target remains unchanged
until the next reboot.
The current default target unit systemd uses to start the system:
#systemctl get-default
List the currently loaded targets:
#systemctl list-units --type target
Configure the system to use a different target unit by default:
#systemctl set-default <name>.target
#systemctl set-default multi-user.target
Verify the default target unit:
#systemctl get-default
Apply the changes by rebooting:
#reboot
CHANGING THE CURRENT TARGET:
On a running system, you can change the target unit in the current boot
without reboot. If you switch to a different target, systemd starts all services
and their dependencies that this target requires, and stops all services that the
new target does not enable.
Isolating a different target affects only the current boot.
Determine the current target:
#systemctl get-default
Change to a different target unit in the current boot:
#systemctl isolate <name>.target
#systemctl isolate multi-user.target
