How do I set up a swap file?

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1

I’ve tried adding this to my configuration.nix:

swapDevices = [ { device = "/swapfile"; size = 2048; } ];

But it gives me the error:

A dependency job for swap.target failed. See 'journalctl -xe' for details.
warning: the following units failed: swapfile.swap

● swapfile.swap - /swapfile
   Loaded: loaded (/etc/fstab; generated)
   Active: failed (Result: exit-code) since Sun 2020-07-26 11:05:53 EDT; 24ms ago
     What: /swapfile
     Docs: man:fstab(5)
           man:systemd-fstab-generator(8)
       IP: 0B in, 0B out
      CPU: 2ms

Jul 26 11:05:53 corpus-db-nixos swapon[18433]: swapon: /swapfile: swapon failed: Invalid argument
Jul 26 11:05:53 corpus-db-nixos systemd[1]: Activating swap /swapfile...
Jul 26 11:05:53 corpus-db-nixos systemd[1]: swapfile.swap: Swap process exited, code=exited, status=255/EXCEPTION
Jul 26 11:05:53 corpus-db-nixos systemd[1]: swapfile.swap: Failed with result 'exit-code'.
Jul 26 11:05:53 corpus-db-nixos systemd[1]: Failed to activate swap /swapfile.
warning: error(s) occurred while switching to the new configuration

And indeed when I try to run sudo swapon /swapfile I get: swapon: /swapfile: swapon failed: Invalid argument.

How should I be setting up a swap file? If it makes a difference, I’m trying to do this on a VPS.

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2

I set up partitions before nixos-install and then it gets generated for me inside hardware-configuration.nix:

  swapDevices =
    [ { device = "/dev/disk/by-uuid/some-redacted-uuid"; }
    ];

(Maybe there are some other ways, too.)

3

There is a bug in the NixOS module, see Swap file created by the module not compatible with Linux 5.7 · Issue #91986 · NixOS/nixpkgs · GitHub.

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4

I can’t seem to get that workaround working for me, either. I tried removing the old swap file, creating a new one using dd, fixing the permissions, and restarting the service, but I’m still getting swapon failed: Invalid argument, no matter what. Furthermore, in journalctl -xe I see kernel: swapon: swapfile has holes.

5

Forgot to mention, the size you use for the dd command needs to exactly match the size set in the module, otherwise the module will call fallocate again:

6

Oh, I didn’t realize it wasn’t a normal swap partition in the OP.

7

NixOS 20.09
Linux Kernel 5.11.1

For those of you who are still having this problem, the following has worked well for me:

The proper way to create a Swap file on BTRFS is Btrfs - ArchWiki, so I set up a oneshot systemd service to create this file for me that is wanted by the NixOS swap-*.service defined in my hardware-configuration.nix file.

This will create the non-compressed, CoW disabled file in my /swap BTRFS subvolume. Then I leave the allocation and enabling of the swap file to NixOS in hardware-configuration.nix

configuration.nix

  systemd.services = {
    create-swapfile = {
      serviceConfig.Type = "oneshot";
      wantedBy = [ "swap-swapfile.swap" ];
      script = ''
        ${pkgs.coreutils}/bin/truncate -s 0 /swap/swapfile
        ${pkgs.e2fsprogs}/bin/chattr +C /swap/swapfile
        ${pkgs.btrfs-progs}/bin/btrfs property set /swap/swapfile compression none
      '';
    };
  };

hardware-configuration.nix

  fileSystems."/swap" = {
    device = "/dev/disk/by-uuid/545353bc-238b-4a5b-bb48-94832272e0b2";
    fsType = "btrfs";
    options = [ "subvol=swap" "compress=lzo" "noatime" ]; # Note these options effect the entire BTRFS filesystem and not just this volume, with the exception of `"subvol=swap"`, the other options are repeated in my other `fileSystem` mounts
  };

  swapDevices = [{
    device = "/swap/swapfile";
    size = (1024 * 16) + (1024 * 2); # RAM size + 2 GB
  }];

As you can see, my nixops deploy succeeds and upon verification the file is enabled and allocated according to my definition.

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8

Thanks for this example! Worked for me with some small tweaks. My goal was enabling hibernation with BTRFS and LUKS.

First needed to run sudo btrfs subvolume create /swap. Then made the following tweaks:

  1. Seems ${pkgs.btrfs-progs}/bin/btrfs property set /swap/swapfile compression none is no longer needed, and in fact it errored out doing this, so I just deleted the line
  2. Skipped adding fileSystems."/swap"
  3. Noticed the create-swapfile service fails once the swap file exists, so added a check into the script:
swapfile="/swap/swapfile"
if [[ -f "$swapfile" ]]; then
  echo "Swap file $swapfile already exists, taking no action"
else
  echo "Setting up swap file $swapfile"
  ${pkgs.coreutils}/bin/truncate -s 0 "$swapfile"
  ${pkgs.e2fsprogs}/bin/chattr +C "$swapfile"
fi

To hibernate to the file, I followed the Arch wiki for calculating the resume_offset. I created a script to automate the end value.

#!/usr/bin/env bash

tmp_dir="$(mktemp -d)"
btrfs_map_physical="$tmp_dir/btrfs_map_physical"
gcc -O2 -o "$btrfs_map_physical" scripts/btrfs_map_physical.c
physical_offset="$(sudo "$btrfs_map_physical" /swap/swapfile | cut -f9 | head -n2 | tail -n1)"
pagesize="$(getconf PAGESIZE)"
resume_offset=$((physical_offset / pagesize))
rm -r "$tmp_dir"
echo "resume_offset=$resume_offset"

Finally I added the following to my hardware-configuration.nix:
boot.kernelParams = ["resume_offset=95450174"];
boot.resumeDevice = "/dev/disk/by-uuid/${rootUUID}";

Final note, this does not seem to work with boot.initrd.systemd.enable = true;.

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