Kernel panic when installl AlmaLinux 9.x on KVM VPS

When installing modern operating systems like AlmaLinux 9 on a KVM VPS, you might encounter a critical "Kernel Panic" error that prevents the server from booting. This article explains the cause and provides a definitive solution.

What is a Kernel Panic?

A Kernel Panic is a critical system error that occurs when the operating system's kernel encounters a problem it cannot recover from. The system halts all operations to prevent further data corruption and displays an error message.

1. The Problem: Kernel Panic After Installation

If your VPS fails to boot after a fresh installation of AlmaLinux 9 and displays an error screen like the one below, the issue is likely an incompatibility with the virtual server's CPU configuration.

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

Kernel panic on AlmaLinux 9 KVM

The primary reason is that AlmaLinux 9 requires modern CPU instruction sets that are not available in KVM's default emulated CPU models (like kvm64 or qemu64). This causes the kernel to fail during initialization, triggering a panic.

2. The Solution: Change the VPS CPU Mode

To resolve this, we need to change the virtual machine's CPU mode within the hypervisor management panel to allow it to use the physical CPU's features. The two recommended modes are:

  • Host Model: This option mirrors the host CPU's model but filters out unsafe features. It provides a good balance between performance and stability.
  • Host Passthrough: This forwards all features of the physical CPU directly to the virtual machine. It offers the highest performance but may limit live migration capabilities.

3. Step-by-Step Instructions

You will need to log into your VPS provider's control panel or your hypervisor's management interface (like Proxmox, SolusVM, or Virtualizor) to perform these steps:

  • Step 1: Shut down your virtual private server (VPS).
  • Step 2: Navigate to your VPS configuration settings, usually under the "Hardware" or "Options" tab.
  • Step 3: Locate and open the CPU settings.
  • Step 4: Under "CPU Type" or "Mode", change the value from the default setting to Host Model or Host Passthrough (often simply labeled "Host").
  • Step 5: Save the changes and start the VPS.

After rebooting, your AlmaLinux 9 virtual server should now boot up successfully without the Kernel Panic error.

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