Swap

Configuration

Swap on NixOS is set with with one of two options swapDevices or zramSwap.enable on /etc/nixos/hardware-configuration.nix.

Add a Swapfile

Add a swapfile with the following :

 swapDevices = [ {
    device = "/var/lib/swapfile";
    size = 16*1024;
  } ];

Size is in megabytes

Disable swap

To remove all swap devices from NixOS, set the following to remove the swap partition or file from being included in /etc/fstab.

swapDevices = lib.mkForce [ ];

If you are using GPT partitioning tables, systemd-gpt-auto-generator(8) will still mount your swap partition automatically. You must therefore turn on attribute 63 on your partition in the partition table. This can be done with gptfdisk or similar:

gdisk /dev/sda
x
a
<partition number>
63
<enter>
w

Enable zram swap

Zram is a kernel module for creating a compressed block device in RAM. The option zramSwap.enable creates such a zram block device and uses it as swap device.

It is an alternative or complementary approach to swap disks, suitable for systems with enough RAM. In the event the system needs to swap it will move uncompressed RAM contents into the compressed area, saving RAM space while effectively increasing the available RAM at the cost of computational power for compression and decompression.

Encrypt swap with random key

Swap can be automatically encrypted with a new key on every boot. This can be used to simplify certain disk layouts, such as securing a swap file on a filesystem partition without an encryption container (such as LUKS).

 swapDevices = [ {
    device = "/dev/sdXY";
    randomEncryption.enable = true; 
  } ];

ZFS and Swap

OpenZFS does not support swap on zvols nor do they support swapfiles on a ZFS dataset.

Instead you should set up a swap partition or swapfile on a non-ZFS filesystem.