Kernel, Modules, Tunables & initrd (Initialisation RAM Disk)
What is the 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.
Where is the Linux Kernel stored?
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 Tunables
In Linux, a tunable refers to a specific type of configuration setting within the kernel. These tunables allow you to customize the behavior of the kernel while the system is running, offering more fine-grained control over how your system operates.
There are lots of tunables set for the Linux Kernel that dictate how the system will handle a variety of system aspects. As an example, there's a tunable for the maximum number of files that a Linux system can have open at any one time, called 'fs-file-max'
View all tunables:
sysctl -a View specific tunable:
sysctl tunable-nameChange tunable value (doesn't persist reboot):
sysctl -w tunable-name=newvaluePermanently change tunable value
Specifically where this can be done is OS dependent, a typical location is /etc/sysctl.d
Create a new file ie 00-custom-settings.conf
contents:
tunable-name=newvalue
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initrd
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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.
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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
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