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

Tips and tricks

Generate bcachefs enabled installation media

Use following Nix expression iso.nix to generate a ISO installation image with a bcachefs enabled kernel

# This module defines a small NixOS installation CD. It does not
# contain any graphical stuff.
{ config, pkgs, lib, ... }:
{
  imports = [
    # Currently fails to build due to ZFS incompatibility with bcachefs
    #<nixpkgs/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix>
    <nixpkgs/nixos/modules/installer/cd-dvd/installation-cd-minimal-new-kernel-no-zfs.nix>
  ];
  boot.supportedFilesystems = [ "bcachefs" ];
  # kernelPackages already defined in installation-cd-minimal-new-kernel-no-zfs.nix
  boot.kernelPackages = lib.mkOverride 0 pkgs.linuxPackages_testing_bcachefs;
  isoImage.squashfsCompression = "gzip -Xcompression-level 1";
}

# nix-build '<nixpkgs/nixos>' -A config.system.build.isoImage -I nixos-config=iso.nix

Continue the installation as usual following the instructions of the NixOS manual. Replace the part where the root filesystem gets formatted and mounted with one of the following two commands:

# mkfs.bcachefs -L nixos /dev/sda1
# mount -t bcachefs /dev/sda1 /mnt

Where /dev/sda1 is the root filesystem partition created for the NixOS installation. Be carefull using the mkfs.bcachefs command, it will erase all data on the given partition.