Why does the system require a reboot to use the newly installed kernel?

Environment

• Red Hat Enterprise Linux 10
• Red Hat Enterprise Linux 9
• Red Hat Enterprise Linux 8
• Red Hat Enterprise Linux 7

Issue

• Why isn't the newly installed kernel being used?
• Why does the system require a reboot to use the newly installed kernel?

Resolution

• After installing a new kernel, it is set as the default kernel, but a reboot is required to load it.
• To reboot the system, run the reboot command with root privilege.
• Services provided by the server will be interrupted by the reboot.

Root Cause

• The kernel is loaded into memory during the boot process and cannot be replaced while the system is running.
• The new kernel requires the bootloader (e.g., GRUB) to load it, which only happens during system startup.

Diagnostic Steps

• Verify the installed kernels:
  • How to list the installed kernel version?
• Verify which kernel is currently loaded:
  • How to check the kernel version in use?
• Verify which kernel is set as the default:
  • How to check the default kernel version?
• Change the default kernel to the one of the installed kernel:
  • How to set the default kernel version?

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