Impermanence

Modules to help you handle persistent state on systems with ephemeral root storage.

The premises of the modules are that you

1. have a root filesystem which somehow gets wiped on reboot - e.g. using tmpfs on /
2. have a mount point where state is kept between reboots
3. want to create links from temporary storage to persistent storage, so that specified files and folders persist between reboots

There are currently two modules: one for NixOS and one for home-manager.

NixOS

To use the module, import it into your configuration with

{
  imports = [ /path/to/impermanence/nixos.nix ];
}

This adds the environment.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:

{
  environment.persistence."/persistent" = {
    hideMounts = true;
    directories = [
      "/var/log"
      "/var/lib/bluetooth"
      "/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 = "/etc/nix/id_rsa"; parentDirectory = { mode = "u=rwx,g=,o="; }; }
    ];
    users.talyz = {
      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"
      ];
    };
  };
}

• ~”/persistent”~ is 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.

• directories are 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=). This also applies to any parent directories which don’t yet exist. Changing this once the directory has been created has no effect.
• files are 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.

  If the file exists in persistent storage, it will be bind mounted to the target path; otherwise it will be symlinked.

• 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.
• users.talyz handles files and directories in talyz’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.

  If the user has a non-standard home directory (i.e. not /home/<username>), the users.<username>.home option has to be set to this path - it can’t currently be automatically deduced due to a limitation in nixpkgs.

Important note: Make sure your persistent volumes are marked with neededForBoot, otherwise you will run into problems.

home-manager

Usage of the home-manager module is very similar to the one of the NixOS module - the key differences are that the persistence option is now under home, rather than environment, and the addition of the submodule option removePrefixDirectory.

Important note: You have to use the home-manager NixOS module (in the nixos directory of home-manager’s repo) in order for this module to work as intended.

To use the module, import it into your configuration with

{
  imports = [ /path/to/impermanence/home-manager.nix ];
}

This 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:

{
  home.persistence."/persistent/home/talyz" = {
    directories = [
      "Downloads"
      "Music"
      "Pictures"
      "Documents"
      "Videos"
      "VirtualBox VMs"
      ".gnupg"
      ".ssh"
      ".nixops"
      ".local/share/keyrings"
      ".local/share/direnv"
      {
        directory = ".local/share/Steam";
        method = "symlink";
      }
    ];
    files = [
      ".screenrc"
    ];
    allowOther = true;
  };
}

• ~”/persistent/home/talyz”~ is the path to your persistent storage location
• directories are all directories you want to link to persistent storage
  • It is possible to switch the linking method between bindfs (the default) and symbolic links.
• files are all files you want to link to persistent storage. These are symbolic links to their target location.
• allowOther allows other users, such as root, to access files through the bind mounted directories listed in directories. Useful for sudo operations, Docker, etc. Requires the NixOS configuration programs.fuse.userAllowOther = true.

Additionally, the home-manager module allows for compatibility with dotfiles repos structured for use with GNU Stow, where the files linked to are one level deeper than where they should end up. This can be achieved by setting removePrefixDirectory to true:

{
  home.persistence."/etc/nixos/home-talyz-nixpkgs/dotfiles" = {
    removePrefixDirectory = true;
    files = [
      "screen/.screenrc"
    ];
    directories = [
      "fish/.config/fish"
    ];
  };
}

In the example, the .screenrc file and .config/fish directory should be linked to from the home directory; removePrefixDirectory removes the first part of the path when deciding where to put the links.

Note: When using bindfs fuse filesystem for directories, the names of the directories you add will be visible in the /etc/mtab file and in the output of mount to all users.

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/ — @etu’s blog post walks the reader through a NixOS-on-tmpfs installation.
• https://grahamc.com/blog/erase-your-darlings — @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.

About the name

Impermanence, also known as the philosophical problem of change, is a
philosophical concept that is addressed in a variety of religions and
philosophies. In Eastern philosophy it is best known for its role in the
Buddhist three marks of existence. It also is an element of Hinduism.