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.[1]
Tips and Tricks
Swapspace
Swapspace is a dynamic swap space manager for GNU/Linux. i.e. it allows unused disk space to be utilised as swap to handle the occasional memory-intensive task, and frees the disk space once done.
Enable it via services.swapspace.enable = true; in your nixos configuration. And after switching, check that systemctl status swapspace.service is green, that's all, swapspace will auto manage swap for you.
See all the options it supports here, search.nixos.org
You can also use zramSwap along with this service.
See your active swap partitions/files with swapon. For eg.
# Read the WARNING above, and adjust 2, 20GB according to your free space
$ # nix shell nixpkgs#stress.out -c stress --vm 2 --vm-bytes 20G
$ swapon
NAME TYPE SIZE USED PRIO
/dev/zram0 partition 13.8G 2.6G 5
/var/lib/swapspace/1 file 5.2G 59.2M -2
/var/lib/swapspace/2 file 6.1G 56.4M -3