Kernel & initrd (Initialisation RAM Disk)
Kernel
====================================================================================
What is a Linux Kernel?
The Linux kernel is the core software that acts as an interface between the hardware and various software applications running on your system.
------------------------------------------------------------------------------------------------------------------------------------------------
Check kernel version
uname -rAfter installing a new kernel version, a server reboot is required.
------------------------------------------------------------------------------------------------------------------------------------------------
Which file in /boot is the kernel?
Typically, the kernel file itself (located within /boot) is prefaced with 'vmlinuz', for example:
vmlinuz-5.15.0-106-generic
Note; is the kernel is prefaced with vmlinuz - this means that the kernel is compressed and must be uncompressed on boot. If the kernel file is prefaced with vmlinux - then it isn't compressed.
Working on Linux systems, you may see systems with various kernels installed.
To check which kernel is currently being treated as the primary one (which will be loaded on boot), you can check the symlinks (in /boot), as below:
vmlinuz -> vmlinuz-5.15.0-107-generic
vmlinuz.old -> vmlinuz-5.15.0-106-generic
------------------------------------------------------------------------------------------------------------------------------------------------
Kernel Modules
What is a kernel module?
A kernel module is essentially a piece of code that can be loaded into the operating system's kernel on demand. Think of it like an extension for the kernel, providing additional functionality without requiring a complete system restart. In other systems, modules are known as drivers.
For clarification; Kernel modules are not the only method that the kernel can be altered. You can also directly edit GRUB to pass additional commands during boot, or alter the kernel manually.
Why do we need kernel modules?
The Linux Kernel is 'monolithic', this means that it's a single file containing every aspect of that particular kernel. In order to change this, we would have to alter and recompile the kernel manually - which is lots of work. Alternatively, we can use pre-built kernel modules to add additional functionality to the Linux Kernel.
About Kernel Modules
Kernel Modules are typically stored in /lib/modules.
Typically, this directory will contain various modules that come pre-installed with the OS, note that these won't all be active.
====================================================================================
initrd
------------------------------------------------------------------------------------------------------------------------------------------------
What is initrd?
initrd (initialisation RAM disk) is essentially a set of instructions used to load the kernel. initrd is stored temporarily in system memory whilst the kernel is loaded.
-----------------------------------------------------------------------------------------------------------------------------------------------
Which file in /boot is initrd?
The initrd file will be prefaced with just that - initrd:
initrd.img-5.15.0-106-generic
To check which initrd file is currently being treated as the primary one (which will be loaded on boot), you can check the symlinks (in /boot), as below:
initrd.img -> initrd.img-5.15.0-107-generic
initrd.img.old -> initrd.img-5.15.0-106-generic
-----------------------------------------------------------------------------------------------------------------------------------------------