Installing NixOS on a new drive

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1

Previously, my system had two physical drives, and I had / and /home mounted on each of them.

The user profile was managed by home-manager (using the NixOS module).

Since my / was getting too full, I purchased a new drive and started moving the system to it. Install was pretty straightforward:

  • Partition the new drive (GPT): create two partitions, fat32 for boot and a filesystem of choice for /.
  • Mount the partitions under /mnt/new/boot and /mnt/new respectively.
  • Copy over /etc/nixos to the new drive.
  • nixos-generate-config on the new drive.
  • Add /boot to the hardware-configuration.nix on the new drive.
  • nixos-install --root /mnt/new

All of this worked as expected, however, I now see there are some issues.

My /home contains symlinks like these:

lrwxrwxrwx 1 ashesh users 45 Jun 22  2019 /home/ashesh/.nix-profile -> /nix/var/nix/profiles/per-user/ashesh/profile

and I’m not sure how to migrate these to the new installation.
I’ve so far found no guides on this and any suggestions will be very helpful. I thought I can just copy over /nix/var/nix/profiles to the new disk but I’m not even sure that’s safe.

How should I proceed?

Edit: This post even talks about copying over a few directories from /var/lib.

2

The .nix-profile link is managed by Nix. You don’t need to copy it over; it’ll get recreated when your user profile is activated in your new installation.

If you have other links pointing to locations under /nix/store, those will be recreated by Home Manager and you don’t need to worry about them either.

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3

That involves having /home mounted and doing nixos-rebuild, correct?

4

Depending on your configuration it might not exist until you install Home Manager, and I don’t actually know if there’s a difference here between installing Home Manager as a module versus using the standalone installation instructions. Ultimately Nix will create what it needs, and we can largely remain ignorant of the details.

5

So, in the end, this turned out to be slightly nontrivial.

I had to also generate the directory as indicated here and then chown it as my user.

Things worked flawlessly afterward.

Steps:

  1. Install new drive into the system.
  2. Create a GPT partition table, with at least two partitions. A fat32 boot partition (labelled boot-new) and a root partition (labelled root-new) with a filesystem of your choice.
  3. Ensure the boot flag is set on the boot partition (without it, in my experience, nixos-install wasn’t installing the bootloader).
  4. Create a new directory for the installation, say /mnt/new.
  5. Mount partitions:
mount /dev/disk/by-label/root-new /mnt/new && \
mkdir /mnt/new/boot  && \
mount /dev/disk/by-label/boot-new /mnt/new/boot
  1. mkdir /mnt/new/etc && cp -r /etc/nixos /mnt/new/etc/
  2. cd /mnt/new
  3. nixos-generate-config and adjust values in filesystems
  4. nixos-install --root /mnt/new

This should install NixOS on the new drive. Once done, reboot and select the new drive from the boot order in BIOS.

Once booted as su, do,

  1. mkdir -m 0755 -p /nix/var/nix/{profiles,gcroots}/per-user/$USER where $USER is your username.
  2. chown -R $USER /nix/var/nix/{profiles,gcroots}/per-user/$USER
  3. nixos-rebuild switch

As usual, with some software, reproducibility goes out of the window. I needed to re-instantiate nextcloud-client and gnome-keyring. Trivial, but doesn’t work out of the box.