Purpose of `swapDevices.*.size`?

musjj · August 13, 2023, 6:50pm

I’m using a swapfile like this:

swapDevices = [ { device = "/swap/swapfile"; } ]

And it’s working just fine, so I’m wondering what’s the purpose of swapDevices.*.size? Is it only necessary if you want NixOS to auto-create the swapfile for you?

elikoga · August 13, 2023, 7:23pm

If you try to set size on a block device (starts with /dev/), you get a warning.

github.com

NixOS/nixpkgs/blob/3fe694c4156b84dac12627685c7ae592a71e2206/nixos/modules/config/swap.nix#L237-L242


      
          warnings =
            concatMap (sw:
              if sw.size != null && hasPrefix "/dev/" sw.device
              then [ "Setting the swap size of block device ${sw.device} has no effect" ]
              else [ ])
            config.swapDevices;

Otherwise a few commands are run in a systemd service to create the swap file just as you said.

github.com

NixOS/nixpkgs/blob/3fe694c4156b84dac12627685c7ae592a71e2206/nixos/modules/config/swap.nix#L264-L275


      
          ${optionalString (sw.size != null) ''
            currentSize=$(( $(stat -c "%s" "$DEVICE" 2>/dev/null || echo 0) / 1024 / 1024 ))
            if [[ ! -b "$DEVICE" && "${toString sw.size}" != "$currentSize" ]]; then
              # Disable CoW for CoW based filesystems like BTRFS.
              truncate --size 0 "$DEVICE"
              chattr +C "$DEVICE" 2>/dev/null || true
          
          
    dd if=/dev/zero of="$DEVICE" bs=1M count=${toString sw.size}
              chmod 0600 ${sw.device}
              ${optionalString (!sw.randomEncryption.enable) "mkswap ${sw.realDevice}"}
            fi
          ''}

And we only create the service if the size (or other conditions) are set.

github.com

NixOS/nixpkgs/blob/3fe694c4156b84dac12627685c7ae592a71e2206/nixos/modules/config/swap.nix#L295


      
                        '';
          
          
            unitConfig.RequiresMountsFor = [ "${dirOf sw.device}" ];
                      unitConfig.DefaultDependencies = false; # needed to prevent a cycle
                      serviceConfig.Type = "oneshot";
                      serviceConfig.RemainAfterExit = sw.randomEncryption.enable;
                      serviceConfig.ExecStop = optionalString sw.randomEncryption.enable "${pkgs.cryptsetup}/bin/cryptsetup luksClose ${sw.deviceName}";
                      restartIfChanged = false;
                    };
          
          
      in listToAttrs (map createSwapDevice (filter (sw: sw.size != null || sw.randomEncryption.enable) config.swapDevices));
          
          
  };
          
          
}

musjj · August 13, 2023, 7:29pm

I see, thanks for clarifying!