NixOS with LUKS, LVM, and Btrfs: A Comprehensive Guide

This guide covers the installation and management of a NixOS system using a full-disk encryption setup with LUKS, LVM for volume management, and Btrfs for the filesystem.

1. 💿 NixOS Installation: Manual Partitioning with LUKS + LVM + Btrfs

This section guides you through a fresh installation of NixOS on a single disk.

Prerequisites

Boot the NixOS installer.
Connect to the internet.
Switch to a root shell: sudo -i.
Identify your target disk: lsblk.
Set a variable for your device:
```
export DEVICE=/dev/sda
```

Step 1. Wipe Disk and Create Partition Table

🚨 Warning: This will destroy all data on the specified disk.

sgdisk --zap-all ${DEVICE}

Step 2. Create Partitions

We will create a standard 4-partition layout for a modern UEFI system.

# Partition 1: 1M BIOS Boot partition (for GRUB compatibility)
sgdisk --new=1:0:+1M --typecode=1:EF02 --change-name=1:boot ${DEVICE}

# Partition 2: 500M EFI System Partition (ESP)
sgdisk --new=2:0:+500M --typecode=2:EF00 --change-name=2:ESP ${DEVICE}

# Partition 3: 4G Swap partition
sgdisk --new=3:0:+4G --typecode=3:8200 --change-name=3:swap ${DEVICE}

# Partition 4: The rest of the disk for our encrypted data
# We use 8E00 which is the typecode for "Linux LVM"
sgdisk --new=4:0:0 --typecode=4:8E00 --change-name=4:root ${DEVICE}

Step 3. Format Unencrypted Filesystems

Format the ESP and swap partitions, giving them labels for easy mounting.

# Format the EFI partition
mkfs.vfat -n ESP ${DEVICE}p2

# Set up the swap partition
mkswap -L swap ${DEVICE}p3

Step 4. Set Up LUKS Encryption and LVM 🔒

This is the core of the setup. We create an encrypted container on our main partition and then build an LVM structure inside it.

# 1. Create the LUKS encrypted container on the fourth partition.
# You will be prompted to enter and confirm a strong passphrase. Remember this!
echo "Formatting the LUKS container. Please enter your encryption passphrase."
cryptsetup luksFormat --label crypted ${DEVICE}p4

# 2. Open the LUKS container to make it accessible.
# This creates a decrypted "virtual" device at /dev/mapper/crypted.
echo "Opening the LUKS container. Please enter your passphrase."
cryptsetup open ${DEVICE}p4 crypted

# 3. Set up LVM *inside* the decrypted container.
# Initialize the physical volume (PV) on the decrypted device
pvcreate /dev/mapper/crypted

# Create the volume group (VG) named "root_vg"
vgcreate root_vg /dev/mapper/crypted

# Create the logical volume (LV) named "root" that uses all available space
lvcreate -l 100%FREE -n root root_vg

Step 5. Format the LVM Volume with Btrfs

Now, we format the LVM logical volume (not the physical partition) with Btrfs.

mkfs.btrfs -L root /dev/root_vg/root

Step 6. Create and Mount Btrfs Subvolumes

We use Btrfs subvolumes to separate parts of our system, which is standard practice for NixOS.

# 1. Mount the top-level Btrfs volume
mount /dev/root_vg/root /mnt

# 2. Create the subvolumes
btrfs subvolume create /mnt/root
btrfs subvolume create /mnt/persist
btrfs subvolume create /mnt/nix

# 3. Unmount the top-level volume
umount /mnt

# 4. Mount the root subvolume with correct options
mount -o subvol=root,compress=zstd,noatime /dev/root_vg/root /mnt

# 5. Create the directories for the other mountpoints
mkdir -p /mnt/persist
mkdir -p /mnt/nix
mkdir -p /mnt/boot

# 6. Mount the other subvolumes
mount -o subvol=persist,noatime,compress=zstd /dev/root_vg/root /mnt/persist
mount -o subvol=nix,noatime,compress=zstd /dev/root_vg/root /mnt/nix

Step 7. Mount Boot Partition and Activate Swap

Finish by mounting the ESP and activating the swap.

# Mount the boot partition
mount ${DEVICE}p2 /mnt/boot

# Activate the swap partition
swapon ${DEVICE}p3

Step 8. Generate NixOS Configuration

Finally, generate the NixOS configuration. The installer will automatically detect the LUKS and LVM setup.

nixos-generate-config --root /mnt

Your /mnt/etc/nixos/hardware-configuration.nix will be auto-generated with the correct LUKS and filesystem entries, similar to this:

# Example /etc/nixos/hardware-configuration.nix

{ config, lib, pkgs, modulesPath, ... }:

{
  imports =
    [ (modulesPath + "/profiles/qemu-guest.nix")
    ];

  boot.initrd.availableKernelModules = [ "ahci" "xhci_pci" "virtio_pci" "sr_mod" "virtio_blk" ];
  boot.initrd.kernelModules = [ "dm-snapshot" ];
  boot.kernelModules = [ "kvm-intel" ];
  boot.extraModulePackages = [ ];

  # This part is automatically added to unlock your disk at boot
  boot.initrd.luks = {
    devices."crypted" = {
      device = "/dev/disk/by-label/crypted";
      preLVM = true;
    };
  };

  # These are your Btrfs subvolumes
  fileSystems."/" =
    { device = "/dev/mapper/root_vg-root";
      fsType = "btrfs";
      options = [ "subvol=root" ];
    };

  fileSystems."/persist" =
    { device = "/dev/mapper/root_vg-root";
      fsType = "btrfs";
      options = [ "subvol=persist" ];
    };

  fileSystems."/nix" =
    { device = "/dev/mapper/root_vg-root";
      fsType = "btrfs";
      options = [ "subvol=nix" ];
    };

  # Your boot and swap partitions
  fileSystems."/boot" = {
      device = "/dev/disk/by-label/ESP";
      fsType = "vfat";
      options = [ "fmask=0022" "dmask=0022" ];
    };

  swapDevices =[ { device = "/dev/disk/by-label/swap"; } ];

  nixpkgs.hostPlatform = lib.mkDefault "x86_64-linux";
}

You can now proceed with editing your configuration.nix and running nixos-install.

2. ➕ Extending an Encrypted LVM Volume with a New Disk

Use this guide when you've added a new physical disk (e.g., /dev/vdb) and want to add its encrypted space to your existing root_vg.

🚨 Pre-flight Check: Backup

Before you begin, ensure you have a backup of any critical data.

Step 1. Partition and Label the New Disk

We'll create a single partition on the new disk (/dev/vdb) and give it a partition label for easy identification.

# Open parted for /dev/vdb
sudo parted /dev/vdb

# Inside parted, run the following commands:
# (parted)
mklabel gpt
mkpart primary 0% 100%
name 1 crypted_ext
quit

This creates /dev/vdb1 and sets its partition name (label) to crypted_ext.

Step 2. Create and Open the LUKS Encrypted Container

Now, encrypt the new partition.

# Encrypt /dev/vdb1.
# -> IMPORTANT: Use the EXACT SAME password as your main encryption.
# -> This allows NixOS to unlock both with a single password prompt.
sudo cryptsetup luksFormat /dev/vdb1

# Open the new LUKS container so we can work with it.
sudo cryptsetup luksOpen /dev/vdb1 crypted_ext_mapper

The unlocked device is now available at /dev/mapper/crypted_ext_mapper.

Step 3. Integrate the New Encrypted Disk into LVM

Add the newly decrypted device as a Physical Volume (PV) to your existing Volume Group (VG).

# 1. Create a new Physical Volume (PV) on the unlocked container.
sudo pvcreate /dev/mapper/crypted_ext_mapper

# 2. Extend your existing 'root_vg' Volume Group with this new PV.
sudo vgextend root_vg /dev/mapper/crypted_ext_mapper

# 3. (Verification) Check your Volume Group. It should now be larger.
sudo vgs

Step 4. Extend the Logical Volume and Btrfs Filesystem

Make the new space available to your filesystem.

# 1. Extend the Logical Volume to use 100% of the new free space.
sudo lvextend -l +100%FREE /dev/mapper/root_vg-root

# 2. Resize the Btrfs filesystem to fill the newly expanded Logical Volume.
sudo btrfs filesystem resize max /

# 3. (Verification) Check your disk space.
df -h /

Step 5. Update `configuration.nix`

This is the most critical step. You must tell NixOS to unlock this second device at boot.

Edit your /etc/nixos/configuration.nix file and add the new device to boot.initrd.luks.

# Your configuration.nix

boot.initrd.luks = {
  devices."crypted" = {
    device = "/dev/disk/by-label/crypted"; # This is your original /dev/vda4
    preLVM = true;
  };

  # --- ADD THIS NEW BLOCK ---
  devices."crypted_ext" = {
    # Use the partition label you set in Step 1
    device = "/dev/disk/by-partlabel/crypted_ext";
    preLVM = true;
    allowDiscards = true; # Good practice for SSDs/VMs
  };
};

Note on Passwords: Because you used the same password for both LUKS devices, NixOS will ask for your password only once at boot and use it to unlock both containers.

Step 6. Rebuild and Reboot

DO NOT REBOOT until you have applied the new configuration.

# Apply your new NixOS configuration
sudo nixos-rebuild switch

# Now it is safe to reboot
sudo reboot

3. ➖ Removing an Encrypted Disk from an LVM Volume

This guide covers the complex process of removing a disk (e.g., /dev/vdb) from a Volume Group when your filesystem spans multiple disks.

🚨 WARNING: This is a high-risk operation. A mistake can lead to total data loss. Back up all critical data before proceeding. This process almost always requires booting from a Live Linux ISO because you cannot shrink a mounted root filesystem.

The Goal

Our goal is to move all data off /dev/vdb (which is part of root_vg) onto your other disk (/dev/mapper/crypted) and then remove /dev/vdb from the LVM setup.

The Problem

You cannot pvmove data off /dev/vdb because there is no free space on the other disk to move it to. You must first shrink your filesystem and logical volume to be smaller than the size of the disk you want to keep.

Example:

Disk 1 (/dev/mapper/crypted): 35G
Disk 2 (/dev/vdb): 20G
Total root_vg size: 55G
Your Goal: You must shrink your Btrfs filesystem and LV to < 35G (e.g., 34G).

Step 1. Boot from a Live Linux ISO

Attach a NixOS, Ubuntu, or other Linux ISO to your VM or machine and boot from it.
Open a terminal.

Step 2. Unlock Encrypted Disks and Activate LVM

# 1. Unlock your *main* encrypted partition (the one you are keeping)
# Replace /dev/vda4 with your actual partition
sudo cryptsetup luksOpen /dev/vda4 crypted

# 2. Unlock the *second* disk's encrypted partition
# (This assumes /dev/vdb is encrypted, following the guide in section 2)
sudo cryptsetup luksOpen /dev/vdb1 crypted_ext

# 3. Activate the LVM Volume Group
sudo vgchange -ay

Step 3. Resize Btrfs and LV (Offline)

This is the most critical part.

# 1. Run a filesystem check (highly recommended)
sudo btrfs check /dev/mapper/root_vg-root

# 2. Shrink the Btrfs filesystem.
# We set it to 34G, which is smaller than our 35G target disk.
sudo btrfs filesystem resize 34G /dev/mapper/root_vg-root

# 3. Shrink the Logical Volume to match.
sudo lvreduce -L 34G /dev/mapper/root_vg-root

Step 4. Reboot into Your Normal System

The offline part is done.

sudo reboot

Remove the Live ISO and boot back into your NixOS. Your system will boot up on a smaller filesystem.

Step 5. Migrate Data and Remove the Disk (Online)

Now that you are back in your system, sudo vgs should show free space in root_vg.

# 1. Load the 'dm-mirror' module, which pvmove needs
sudo modprobe dm_mirror

# 2. Move all data extents off the disk you want to remove.
# This will move data from crypted_ext to the free space on crypted.
sudo pvmove -v /dev/mapper/crypted_ext_mapper

# 3. Remove the now-empty Physical Volume from the Volume Group.
sudo vgreduce root_vg /dev/mapper/crypted_ext_mapper

# 4. Remove the LVM metadata from the device.
sudo pvremove /dev/mapper/crypted_ext_mapper

Step 6. Update `configuration.nix` and Clean Up

Edit your /etc/nixos/configuration.nix and remove the entry for crypted_ext from boot.initrd.luks.
Rebuild your system:
```
sudo nixos-rebuild switch
```
You can now safely close the LUKS container and reboot. The disk /dev/vdb is completely free.
```
sudo cryptsetup luksClose crypted_ext_mapper
sudo reboot
```

4. 📚 LUKS Command Reference

Common cryptsetup commands for managing LUKS devices.

Format a new LUKS container:

# --label is recommended for use in /dev/disk/by-label/
cryptsetup luksFormat --label crypted /dev/sda4

Open (decrypt) a container:

# This creates a device at /dev/mapper/my_decrypted_volume
cryptsetup luksOpen /dev/sda4 my_decrypted_volume

Close (lock) a container:

cryptsetup luksClose my_decrypted_volume

Add a new password (key slot):

# You will be prompted for an *existing* password first.
cryptsetup luksAddKey /dev/sda4

Remove a password:

# You will be prompted for the password you wish to remove.
cryptsetup luksRemoveKey /dev/sda4

View header information (and key slots):
```
cryptsetup luksDump /dev/sda4
```
Resize an online LUKS container: (Useful if you resize the underlying partition).
```
cryptsetup resize my_decrypted_volume
```

5. 📚 LVM Command Reference

Common commands for managing LVM.

Physical Volume (PV) - The Disks

Initialize a disk for LVM:
```
pvcreate /dev/mapper/crypted
```
List physical volumes:
```
pvs
pvdisplay
```

Move data from one PV to another (within the same VG):

# Moves all data *off* /dev/sdb1
pvmove /dev/sdb1

# Moves data from /dev/sdb1 *to* /dev/sdc1
pvmove /dev/sdb1 /dev/sdc1

Remove LVM metadata from a disk:

# Only run this *after* removing the PV from its VG.
pvremove /dev/sdb1

Volume Group (VG) - The Pool of Disks

Create a new VG:

# Creates a VG named "my_vg" using two disks
vgcreate my_vg /dev/sdb1 /dev/sdc1

List volume groups:
```
vgs
vgdisplay
```
Add a disk (PV) to an existing VG:
```
vgextend my_vg /dev/sdd1
```

Remove a disk (PV) from a VG:

# The PV must be empty (use pvmove first).
vgreduce my_vg /dev/sdb1

Remove a VG:

# Make sure all LVs are removed first.
vgremove my_vg

Logical Volume (LV) - The "Partitions"

Create a new LV:

# Create a 50G LV named "my_lv" from the "my_vg" pool
lvcreate -L 50G -n my_lv my_vg

# Create an LV using all remaining free space
lvcreate -l 100%FREE -n my_other_lv my_vg

List logical volumes:
```
lvs
lvdisplay
```

Extend an LV (and its filesystem):

# Extend the LV to be 100G in total
lvresize -L 100G /dev/my_vg/my_lv

# Add 20G to the LV's current size
lvresize -L +20G /dev/my_vg/my_lv

# Extend the LV to use all free space in the VG
lvextend -l +100%FREE /dev/my_vg/my_lv

# --- IMPORTANT ---
# After extending, you must resize the filesystem inside it.
# For ext4:
resize2fs /dev/my_vg/my_lv
# For btrfs:
btrfs filesystem resize max /path/to/mountpoint

Reduce an LV (and its filesystem): 🚨 DANGEROUS! You must shrink the filesystem first.

# 1. Shrink the filesystem (e.g., ext4, UNMOUNTED)
resize2fs /dev/my_vg/my_lv 40G

# 2. Shrink the LV to match
lvreduce -L 40G /dev/my_vg/my_lv

# For Btrfs, you can often do it online:
# 1. Shrink Btrfs
btrfs filesystem resize 40G /path/to/mountpoint
# 2. Shrink LV
lvreduce -L 40G /dev/my_vg/my_lv

Remove an LV:

# Make sure it's unmounted first.
lvremove /dev/my_vg/my_lv

6. 📚 Btrfs Command Reference

Common commands for managing Btrfs filesystems and subvolumes.

Format a device:

# -L sets the label
mkfs.btrfs -L root /dev/my_vg/my_lv

Resize a filesystem:

# Grow to fill the maximum available space (after an lvextend)
btrfs filesystem resize max /path/to/mountpoint

# Set to a specific size (e.g., 50G)
btrfs filesystem resize 50G /path/to/mountpoint

# Shrink by 10G
btrfs filesystem resize -10G /path/to/mountpoint

Show filesystem usage:

# Btrfs-aware 'df'
btrfs filesystem df /path/to/mountpoint

Create a subvolume:

# Mount the top-level (ID 5) volume first
mount /dev/my_vg/my_lv /mnt

# Create subvolumes
btrfs subvolume create /mnt/root
btrfs subvolume create /mnt/nix

umount /mnt

List subvolumes:

btrfs subvolume list /path/to/mountpoint

Delete a subvolume:

# Deleting a subvolume is recursive and instant
btrfs subvolume delete /mnt/nix

Create a snapshot:

# Create a read-only snapshot of 'root'
btrfs subvolume snapshot -r /mnt/root /mnt/root-snapshot

# Create a writable snapshot (a clone)
btrfs subvolume snapshot /mnt/root /mnt/root-clone

Check a Btrfs filesystem (unmounted):
```
btrfs check /dev/my_vg/my_lv
```

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