Transitioning to nixos‑rebuild switch: Unresolved initrd regeneration on kernel updates

Answer the question first

By default nixos-rebuild switch does not rebuild the initrd when a kernel package is upgraded. The tool only triggers a new initrd if it detects a change in the kernel package itself or in the initrd‑generation options. If the kernel upgrade is considered “in‑place” (e.g. the same package name, only a new version), the existing initrd is reused, which can leave the initrd out of sync with the running kernel and cause unresolved initrd errors during boot.

Likely explanation

The initrd is a snapshot of the kernel and its modules. NixOS tracks the derivation that produced the current initrd. When nixos-rebuild switch runs, it compares the requested kernel package against the one that was used to build the current initrd. If the derivation hash is identical, the rebuild process assumes the initrd is still valid and skips regeneration. This optimisation keeps rebuild times short but can lead to boot failures if the kernel modules changed in the new package version.

Confirmed facts

• Initrd regeneration occurs only when the kernel derivation or initrd options change.
• Running nixos-rebuild switch without any flags will reuse the existing initrd unless a change is detected.
• Explicit options such as --initrd or --upgrade force a new initrd to be generated.
• Configuration options boot.initrd.isSafeToUse, boot.initrd.enable, and boot.initrd.systemd.enable control whether the initrd is considered safe to reuse.

Steps to resolve the issue in this case

1. Verify the kernel and initrd you are currently using:

   uname -r
   ls -l /boot/initrd-$(uname -r).img
   

2. Force a new initrd to be built:

   nixos-rebuild switch --upgrade
   # or
   nixos-rebuild switch --initrd
   

3. Ensure the initrd generation hooks are enabled in /etc/nixos/configuration.nix:

   boot.initrd.enable = true;
   boot.initrd.systemd.enable = true;
   

4. Rebuild and switch:

   nixos-rebuild switch
   

5. Reboot and check for unresolved initrd messages:

   dmesg | grep initrd
   

6. Verify that the new initrd contains the updated kernel modules:

   nix-store --query --requisites /nix/store/...initrd-*.img | grep kernel
   

What to ask next

To tailor the recommendation further, could you tell me which kernel package was upgraded (e.g. kernel, kernel-modules, kernelPackages) and whether the initrd generation hook (boot.initrd.enable) was active during the rebuild?

Practical verification

After the rebuild, inspect /var/log/nixos-rebuild.log or journalctl -b -1 for a line such as initrd generation skipped. If present, the initrd was not regenerated; if absent, a new initrd was built.