Bcachefs

Bcachefs is a next-generation CoW filesystem that aims to provide features from Btrfs and ZFS with a cleaner codebase, more stability, greater speed and a GPL-compatible license. It is built upon Bcache and is mainly developed by Kent Overstreet.

Installation

To enable filesystem support and availability of user-space utils, add following line to the system configuration

/etc/nixos/configuration.nix

boot.supportedFilesystems = [ "bcachefs" ];

Usage

Format and mount a single device

# bcachefs format /dev/sda
# mount -t bcachefs /dev/sda /mnt

Format drive with encryption enabled, unlock and mount it afterwards. Following bcachefs commands will ask for a password:

# bcachefs format --encrypt /dev/sda
# bcachefs unlock /dev/sda
# mount -t bcachefs /dev/sda /mnt

Create a subvolume of a mounted bcachefs filesystem. The snapshot of the filesystem state is accessible in the directory /mnt/snap1.

# bcachefs subvolume snapshot /mnt /mnt/snap1

Configuration

Change encryption password for Bcachefs formatted device /dev/sda1

# bcachefs set-passphrase /dev/sda1

Enable zstd compression for device /dev/sda1

/etc/nixos/hardware-configuration.nix

fileSystems."/" =
{ device = "/dev/sda1";
  fsType = "bcachefs";
  options = [ "compression=zstd" ];
};

Tips and tricks

Generate bcachefs enabled installation media

Use following Nix Flake-expression to generate a ISO installation image with a bcachefs enabled kernel

flake.nix

{
  description = "Bcachefs enabled installation media";
  inputs.nixos.url = "nixpkgs/nixos-23.11";
  outputs = { self, nixos }: {
    nixosConfigurations = {
      exampleIso = nixos.lib.nixosSystem {
        system = "x86_64-linux";
        modules = [
          "${nixos}/nixos/modules/installer/cd-dvd/installation-cd-minimal-new-kernel-no-zfs.nix"
          ({ lib, pkgs, ... }: {
            boot.supportedFilesystems = [ "bcachefs" ];
            boot.kernelPackages = lib.mkOverride 0 pkgs.linuxPackages_testing;
          })
        ];
      };
    };
  };
}

The following commands will generate the iso-image which will be available in the directory ./result/iso

# git init
# git add flake.nix
# nix build .#nixosConfigurations.exampleIso.config.system.build.isoImage

NixOS installation on bcachefs

Using the installation media generated above, continue the installation as usual following the instructions of the NixOS manual.

For a UEFI installation, the partitioning needs to be adjusted as following

Note: Be sure on which disk you'll perform these filesystem operations. All existing data on it will be erased.

# parted /dev/sda -- mklabel gpt
# parted /dev/sda -- mkpart ESP fat32 1MB 512MB
# parted /dev/sda -- set 1 esp on
# parted /dev/sda -- mkpart primary 512MB 100%

Formatting the boot partition /dev/sda1 and the root filesystem /dev/sda2

# mkfs.fat -F 32 -n boot /dev/sda1
# mkfs.bcachefs -L nixos /dev/sda2

In case you want to enable filesystem encryption, there's a workaround for a bug affecting NixOS 23.05. Formatting and unlocking the encrypted partition would look like this

# nix-env -iA nixos.keyutils
# keyctl link @u @s
# bcachefs format --encrypt /dev/sda2
# bcachefs unlock /dev/sda2

Mount filesystems

# mount /dev/sda2 /mnt
# mkdir /mnt/boot
# mount /dev/disk/by-label/boot /mnt/boot

Continue installation as recommended by the NixOS manual.

Before nixos-install you'll have to adapt the generated NixOS configuration to also use the latest Linux kernel supporting Bcachefs. Add the following line to the configuration.nix in your installation root (/mnt):

/mnt/etc/nixos/configuration.nix

boot.kernelPackages = pkgs.linuxPackages_latest;