#+TITLE: Impermanence Lets you choose what files and directories you want to keep between reboots - the rest are thrown away. Why would you want this? - It keeps your system clean by default. - It forces you to declare settings you want to keep. - It lets you experiment with new software without cluttering up your system. There are a few different things to set up for this to work: - A root filesystem which somehow gets wiped on reboot. There are a few ways to achieve this. See the [[#system-setup][System setup]] section for more info. - At least one mounted volume where the files and directories you want to keep are stored permanently. - At least one of the modules in this repository, which take care of linking or bind mounting files between the persistent storage mount point and the root file system. See the [[#module-usage][Module usage]] section for more info. * Contact Join the [[https://matrix.to/#/#impermanence:nixos.org][matrix room]] to chat about the project. * System setup There are many ways to wipe your root partition between boots. This section lists a few common ways to accomplish this, but is by no means an exhaustive list. *** tmpfs The easiest method is to use a tmpfs filesystem for the root. This is the easiest way to set up impermanence on systems which currently use a traditional filesystem (ext4, xfs, etc) as the root filesystem, since you don't have to repartition. All data stored in tmpfs only resides in system memory, not on disk. This automatically takes care of cleaning up between boots, but also comes with some pretty significant drawbacks: - Downloading big files or trying programs that generate large amounts of data can easily result in either an out-of-memory or disk-full scenario. - If the system crashes or loses power before you've had a chance to move files you want to keep to persistent storage, they're gone forever. Using tmpfs as the root filesystem, the filesystem setup would look something like this: #+begin_src nix { fileSystems."/" = { device = "none"; fsType = "tmpfs"; options = [ "defaults" "size=25%" "mode=755" ]; }; fileSystems."/persistent" = { device = "/dev/root_vg/root"; neededForBoot = true; fsType = "btrfs"; options = [ "subvol=persistent" ]; }; fileSystems."/nix" = { device = "/dev/root_vg/root"; fsType = "btrfs"; options = [ "subvol=nix" ]; }; fileSystems."/boot" = { device = "/dev/disk/by-uuid/XXXX-XXXX"; fsType = "vfat"; }; } #+end_src where the ~size~ option determines how much system memory is allowed to be used by the filesystem. *** BTRFS subvolumes A more advanced solution which doesn't have the same drawbacks as using tmpfs is to use a regular filesystem, but clean it up between boots. A relatively easy way to do this is to use BTRFS and create a new subvolume to use as root on boot. This also allows you to keep a number of old roots around, in case of crashes, power outages or other accidents. A setup which would automatically remove roots that are older than 30 days could look like this: #+begin_src nix { utils, ... }: { fileSystems."/" = { device = "/dev/root_vg/root"; fsType = "btrfs"; options = [ "subvol=root" ]; }; boot.initrd.systemd = { enable = true; # Default in 26.05 services.wipe-root = { # Specify dependencies explicitly unitConfig.DefaultDependencies = false; # The script needs to run to completion before this service is done serviceConfig.Type = "oneshot"; # This service is required for boot to succeed wantedBy = [ "initrd.target" ]; # Should complete before any file systems are mounted before = [ "sysroot.mount" ]; # Wait for the disk to appear requires = [ "${utils.escapeSystemdPath "/dev/root_vg/root"}.device" ]; after = [ "${utils.escapeSystemdPath "/dev/root_vg/root"}.device" # Allow hibernation to resume before trying to alter any data "local-fs-pre.target" ]; script = '' mkdir /btrfs_tmp mount /dev/root_vg/root /btrfs_tmp trap 'umount /btrfs_tmp' EXIT if [[ -e /btrfs_tmp/root ]]; then mkdir -p /btrfs_tmp/old_roots timestamp=$(date --date="@$(stat -c %Y /btrfs_tmp/root)" "+%Y-%m-%-d_%H:%M:%S") mv /btrfs_tmp/root "/btrfs_tmp/old_roots/$timestamp" fi for i in $(find /btrfs_tmp/old_roots/ -maxdepth 1 -mtime +30); do btrfs subvolume delete --recursive "$i" done btrfs subvolume create /btrfs_tmp/root ''; }; }; fileSystems."/persistent" = { device = "/dev/root_vg/root"; neededForBoot = true; fsType = "btrfs"; options = [ "subvol=persistent" ]; }; fileSystems."/nix" = { device = "/dev/root_vg/root"; fsType = "btrfs"; options = [ "subvol=nix" ]; }; fileSystems."/boot" = { device = "/dev/disk/by-uuid/XXXX-XXXX"; fsType = "vfat"; }; } #+end_src This assumes the BTRFS filesystem can be found in an LVM volume group called ~root_vg~. Adjust the path as necessary. * Module usage There are currently two modules: one for NixOS and one for Home Manager. *** NixOS To use the module, import it into your configuration with #+begin_src nix { imports = [ /path/to/impermanence/nixos.nix ]; } #+end_src or use the provided ~nixosModules.impermanence~ flake output: #+begin_src nix { inputs = { impermanence.url = "github:nix-community/impermanence"; }; outputs = { self, nixpkgs, impermanence, ... }: { nixosConfigurations.sythe = nixpkgs.lib.nixosSystem { system = "x86_64-linux"; modules = [ impermanence.nixosModules.impermanence ./machines/sythe/configuration.nix ]; }; }; } #+end_src This adds the ~environment.persistence~ option, which is an attribute set of submodules, where the attribute name is the path to persistent storage. Note that this project has ~nixpkgs~ and ~home-manager~ as dependencies for development, but these are not needed when using the module. If these extra dependencies appearing in your ~flake.lock~ bother you, you can remove them like this: #+begin_src nix inputs = { impermanence.url = "github:nix-community/impermanence"; impermanence.inputs.nixpkgs.follows = ""; impermanence.inputs.home-manager.follows = ""; }; #+end_src Usage is shown best with an example: #+begin_src nix { environment.persistence."/persistent" = { enable = true; # NB: Defaults to true, not needed hideMounts = true; directories = [ "/var/log" "/var/lib/bluetooth" "/var/lib/nixos" "/var/lib/systemd/coredump" "/etc/NetworkManager/system-connections" { directory = "/var/lib/colord"; user = "colord"; group = "colord"; mode = "u=rwx,g=rx,o="; } ]; files = [ "/etc/machine-id" { file = "/var/keys/secret_file"; parentDirectory = { mode = "u=rwx,g=,o="; }; } ]; users.bird = { directories = [ "Downloads" "Music" "Pictures" "Documents" "Videos" "VirtualBox VMs" { directory = ".gnupg"; mode = "0700"; } { directory = ".ssh"; mode = "0700"; } { directory = ".nixops"; mode = "0700"; } { directory = ".local/share/keyrings"; mode = "0700"; } ".local/share/direnv" ]; files = [ ".screenrc" ]; }; }; } #+end_src ***** ~"/persistent"~ The path to your persistent storage location. This allows for multiple different persistent storage locations. If you, for example, have one location you back up and one you don't, you can use both by defining two separate attributes under ~environment.persistence~. - ~enable~ determines whether the persistent storage location should be enabled or not. Useful when sharing configurations between systems with and without impermanence setups. Defaults to ~true~. - ~hideMounts~ allows you to specify whether to hide the bind mounts from showing up as mounted drives in the file manager. If enabled, it sets the mount option ~x-gvfs-hide~ on all the bind mounts. - ~allowTrash~ allows you to specify whether to allow apps that use certain GTK-based technologies such as GIO to put items in the trash. If enabled, it sets the mount option ~x-gvfs-trash~ on all the bind mounts. ****** ~directories~ All directories you want to bind mount to persistent storage. A directory can be represented either as a string, simply denoting its path, or as a submodule. The submodule representation is useful when the default assumptions, mainly regarding permissions, are incorrect. The available options are: - ~directory~, the path to the directory you want to bind mount to persistent storage. Only setting this option is equivalent to the string representation. - ~persistentStoragePath~, the path to persistent storage. Defaults to the ~environment.persistence~ submodule name, i.e. ~"/persistent"~ in the example. This should most likely be left to its default value - don't change it unless you're certain you really need to. - ~user~, the user who should own the directory. If the directory doesn't already exist in persistent storage, it will be created and this user will be its owner. This also applies to any parent directories which don't yet exist. Changing this once the directory has been created has no effect. - ~group~, the group who should own the directory. If the directory doesn't already exist in persistent storage, it will be created and this group will be its owner. This also applies to any parent directories which don't yet exist. Changing this once the directory has been created has no effect. - ~mode~, the permissions to set for the directory. If the directory doesn't already exist in persistent storage, it will be created with this mode. Can be either an octal mode (e.g. ~0700~) or a symbolic mode (e.g. ~u=rwx,g=,o=~). Parent directories that don't yet exist are created with default permissions. Changing this once the directory has been created has no effect. ****** ~files~ All files you want to link or bind to persistent storage. A file can be represented either as a string, simply denoting its path, or as a submodule. The submodule representation is useful when the default assumptions, mainly regarding the permissions of its parent directory, are incorrect. The available options are: - ~file~, the path to the file you want to bind mount to persistent storage. Only setting this option is equivalent to the string representation. - ~persistentStoragePath~, the path to persistent storage. Defaults to the ~environment.persistence~ submodule name, i.e. ~"/persistent"~ in the example. This should most likely be left to its default value - don't change it unless you're certain you really need to. - ~parentDirectory~, the permissions that should be applied to the file's parent directory, if it doesn't already exist. Available options are ~user~, ~group~ and ~mode~. See their definition in ~directories~ above. - ~method~, the method used to link the file to persistent storage. ~"auto"~, the default, uses a bind mount when the file exists in persistent storage and a symlink otherwise. ~"symlink"~ uses a symlink regardless. Only change this to ~"symlink"~ if ~"auto"~ is giving you issues. ****** ~users.bird~ Handles files and directories in ~bird~'s home directory. The ~users~ option defines a set of submodules which correspond to the users' names. The ~directories~ and ~files~ options of each submodule work like their root counterparts, but the paths are automatically prefixed with with the user's home directory. /Important note:/ Make sure all your persistent and ephemeral storage volumes are marked with ~neededForBoot~, otherwise you will run into problems. *** Home Manager The usage of the Home Manager module is very similar to the one of the NixOS module - the key difference is that the ~persistence~ option is now under ~home~, rather than ~environment~. /Important note:/ You have to use the Home Manager NixOS module (in the ~nixos~ directory of Home Manager's repo) *and* the NixOS ~persistence~ module in order for this module to work as intended. The module will be loaded automatically when you [[#nixos][import the NixOS persistence module]] and the Home Manager NixOS module. It adds the ~home.persistence~ option, which is an attribute set of submodules, where the attribute name is the path to persistent storage. Usage is shown best with an example: #+begin_src nix { inputs = { home-manager.url = "github:nix-community/home-manager"; impermanence.url = "github:nix-community/impermanence"; }; outputs = { home-manager, nixpkgs, impermanence, ... }: { nixosConfigurations.sythe = nixpkgs.lib.nixosSystem { system = "x86_64-linux"; modules = [ { imports = [ impermanence.nixosModules.impermanence home-manager.nixosModules.home-manager ]; home-manager.users.bird = { home.persistence."/persistent" = { directories = [ "Downloads" "Music" "Pictures" "Documents" "Videos" "VirtualBox VMs" { directory = ".gnupg"; mode = "0700"; } { directory = ".ssh"; mode = "0700"; } { directory = ".nixops"; mode = "0700"; } { directory = ".local/share/keyrings"; mode = "0700"; } ".local/share/direnv" ]; files = [ ".screenrc" ]; }; }; } ]; }; }; } #+end_src ****** ~"/persistent"~ The path to your persistent storage location. ****** ~directories~ All directories you want to bind mount to persistent storage. See [[#directories][the NixOS module directories section]] for more details. ****** ~files~ All files you want to link or bind to persistent storage. See [[#files][the NixOS module files section]] for more details. ** Further reading The following blog posts provide more information on the concept of ephemeral roots: - https://elis.nu/blog/2020/05/nixos-tmpfs-as-root/ --- [[https://github.com/etu/][@etu]]'s blog post walks the reader through a NixOS-on-tmpfs installation. - https://grahamc.com/blog/erase-your-darlings --- [[https://github.com/grahamc/][@grahamc]]'s blog post details why one would want to erase their state at every boot, as well as how to achieve this using ZFS snapshots. - https://willbush.dev/blog/impermanent-nixos/ --- [[https://github.com/willbush/][@willbush]]'s blog post provides a detailed NixOS-on-tmpfs guide with optional LUKS encryption, and utilizing nix flakes for an opinionated install.