Ephemeral system with NixOS

Recently, I discovered the following reddit post and blog posts:

I thought I didn't need any kind of persistence tool, because btrfs is awesome. So I decided to give it a go myself.

In my case, I'm using btrfs so it's very easy to get going with.

I went through and decided I wanted to keep:

/nix (of course, I don't want to have to download everything all the time)
/home (for now, I'll have a go at that one later)
/var/lib (databases, docker, etc)
/etc/ssh (host keys)
/etc/NetworkManager (system connections, VPNs etc)

And I was content with the rest being recreated on boot, especially /tmp, because it vastly speeds up a lot of things!

Firstly I converted the directories I wanted to keep into subvolumes (in rescue mode or live/other OS):

mv /dir-to-keep /dir2
btrfs su c /dir-to-keep
cp -rpv --reflink=always /dir2/* /dir-to-keep/
rm -rf /dir2

--reflink=always makes a CoW copy in btrfs, so this is a much faster way to copy over data.

This worked for most of my directories, except for /nix, so I had to use --reflink=auto because I had a lot of hard links due to nix-store optimisation, so it'll CoW only that which it can.

Noteworthy is that to do /nix to avoid commands disappearing involves doing this (optionally from the running system):

btrfs su c /nix2
cp -rpv --reflink=auto /nix/* /nix2/ # Many hardlinks may be duplicated!

and then from a live or other system (because moving things in-use is dangerous and may not actually work):

mount /dev/XXX /mnt/root
rm -rf /mnt/root/nix
mv /mnt/root/nix2 /mnt/root/nix

Finally I'm able to have / as a tmpfs!

So I added the following to hardware-configuration.nix:

{
  fileSystems."/" =
    {
      device = "tmpfs";
      fsType = "tmpfs";
      options = [
        "size=2G"
      ];
    };

  fileSystems."/nix" =
    {
      device = "/dev/XXX";
      fsType = "btrfs";
      options = [
        "subvol=/nix"
        "noatime"
      ];
    };

  fileSystems."/etc/ssh" =
    {
      device = "/dev/XXX";
      fsType = "btrfs";
      options = [
        "subvol=/etc/ssh"
        "noatime"
      ];
    };

  fileSystems."/etc/NetworkManager" =
    {
      device = "/dev/XXX";
      fsType = "btrfs";
      options = [
        "subvol=/etc/NetworkManager"
        "noatime"
      ];
    };

  fileSystems."/var/lib" =
    {
      device = "/dev/XXX";
      fsType = "btrfs";
      options = [
        "subvol=/var/lib"
        "noatime"
      ];
    };

  fileSystems."/home" =
    {
      device = "/dev/XXX";
      fsType = "btrfs";
      options = [
        "subvol=/home"
        "noatime"
      ];
    };
}

and making sure my passwords were immutable and persistent via the clauses in configuration.nix:

{
  users.mutableUsers = false;
  users.users.root.initialHashedPassword = "$6$8RZ1PPxKU6h$dNHnIWiq.h8s.7SpMW14FzK9bJwg1f6Mt.972/2Fij4zPrhR0X4m3JTNPtGAyeMKZk3I8x/Xro.vJolwVvwd9.";
  users.users.dwd.initialHashedPassword = "$6$EDn9CboEV/$ESAQifZD0wiVkYf1MuyLqs.hP7mvelpoPnSGEI7CmwuUifi090PT6FQqHsdhlZSXSlqrT9EH.mIfUvxPCA5q.1";
}

(hey, they're hashed and SHA-512'd, do you want to try to crack them?)

Triggering a rebuild and a reboot (to actually apply / as tmpfs), and success! Hooray!

I can now delete everything from the root subvolume that I didn't make into a subvolume.

tree -x from the root subvolume now looks like:

$ tree -x
.
├── etc
│   ├── NetworkManager
│   └── ssh
├── home
├── nix
└── var
    └── lib

so I can share these with another OS if I so wish.

I'm currently investigating doing the same to /home, so I'll keep you updated.

Till next time!

Comments

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Ephemeral system with NixOS

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