Add ZRAM for Kexec on Systems with Limited RAM

[Fijxu]

Prerequisites

• I have searched existing issues to make sure this isn't a duplicate
• I have checked the documentation to see if this feature already exists

Problem Description

Using nixos-anywhere on systems with 1GB of RAM almost always crashes due to out of memory, so I thought of using ZRAM (zstd compressed) on one of my 1GB RAM servers to see if I was able to install it NixOS on it without using --no-disko-deps and it actually worked, leaving my server with approximately 50MB of RAM available during the NixOS installation.

Proposed Solution

Add a command line option to define usage of ZRAM for people that want to use nixos-anywhere on 1GB systems.

Alternatives Considered

No response

Feature Type

New command-line option

Use Case

No response

Implementation Ideas

No response

Additional Context

No response

[hasirchan]

I also attempted installing NixOS using nixos-anywhere on a budget VPS with approximately 1GB (reported as 965.5 MiB) of memory.

Without the --no-disko-deps option, the installation occasionally completed without crashing. Even with --no-disko-deps enabled, failures still occurred sometimes and not due to triggering the OOM Killer, but rather freezing completely during the copying files from cache.nixos.org.

Subsequently, I modified src/nixos-anywhere.sh to enable zram after entering kexec and set its size to 50% of physical memory. Even without the --no-disko-deps option, the installation succeeded. I tested this three times, all with success.

I also tested multiple times on a VPS with 512MB (reported as 475.7 MiB) of memory, where it failed to enter kexec with the error kexec_load failed: Invalid argument. It remains unclear whether a VPS with more memory (e.g., 768MB) could successfully enter kexec.

Although maybe further testing is needed, I believe "enabling zram after successfully entering kexec" as an option would be highly beneficial for low-memory VPS.

[DavHau]

@hasirchan Sounds like a good change. Could you please share your modification to nixos-anywhere.sh or just open a PR?

[kidonng]

I asked AI (Mimo V2 Pro) to generate a patch and was able to use it (along with --no-disko-deps) to pass disk partitioning on a 1GB RAM machine, but cannot finish building the system closure. If it helps:

diff --git a/src/nixos-anywhere.sh b/src/nixos-anywhere.sh
index bf62667..b0e8ca7 100755
--- a/src/nixos-anywhere.sh
+++ b/src/nixos-anywhere.sh
@@ -833,6 +833,24 @@ EOF
   fi
 }
 
+setupZram() {
+  step "Setting up zram swap for low-memory VPS"
+  runSsh bash <<'ZRAM'
+    modprobe zram 2>/dev/null || true
+    if [ -b /dev/zram0 ]; then
+      mem_total=$(grep MemTotal /proc/meminfo | awk '{print $2}')
+      zram_size=$((mem_total * 1024 / 2))
+      echo zstd > /sys/block/zram0/comp_algorithm 2>/dev/null || true
+      echo $zram_size > /sys/block/zram0/disksize
+      mkswap /dev/zram0
+      swapon /dev/zram0
+      echo "zram swap enabled: $(cat /proc/swaps | grep zram)"
+    else
+      echo "zram module not available, skipping"
+    fi
+ZRAM
+}
+
 runDisko() {
   local diskoScript=$1
   for path in "${!diskEncryptionKeys[@]}"; do
@@ -1001,6 +1019,7 @@ main() {
 
   if [[ ${phases[kexec]} == 1 ]]; then
     runKexec
+    setupZram
   fi
 
   if [[ ${hardwareConfigBackend} != "none" ]]; then

[dot-file]

I asked AI (Mimo V2 Pro) to generate a patch and was able to use it (along with --no-disko-deps) to pass disk partitioning on a 1GB RAM machine, but cannot finish building the system closure. If it helps:

Just tested with zram size being the same as memory size and it worked twice. I also set 1200MB zram size on the original OS before running nixos-anywhere just in case. (UPD: Worked without it just fine)

Here's the version of the patch I used (I just removed / 2):

diff --git a/src/nixos-anywhere.sh b/src/nixos-anywhere.sh
index bf62667..b0e8ca7 100755
--- a/src/nixos-anywhere.sh
+++ b/src/nixos-anywhere.sh
@@ -833,6 +833,24 @@ EOF
   fi
 }
 
+setupZram() {
+  step "Setting up zram swap for low-memory VPS"
+  runSsh bash <<'ZRAM'
+    modprobe zram 2>/dev/null || true
+    if [ -b /dev/zram0 ]; then
+      mem_total=$(grep MemTotal /proc/meminfo | awk '{print $2}')
+      zram_size=$((mem_total * 1024))
+      echo zstd > /sys/block/zram0/comp_algorithm 2>/dev/null || true
+      echo $zram_size > /sys/block/zram0/disksize
+      mkswap /dev/zram0
+      swapon /dev/zram0
+      echo "zram swap enabled: $(cat /proc/swaps | grep zram)"
+    else
+      echo "zram module not available, skipping"
+    fi
+ZRAM
+}
+
 runDisko() {
   local diskoScript=$1
   for path in "${!diskEncryptionKeys[@]}"; do
@@ -1001,6 +1019,7 @@ main() {
 
   if [[ ${phases[kexec]} == 1 ]]; then
     runKexec
+    setupZram
   fi
 
   if [[ ${hardwareConfigBackend} != "none" ]]; then

[cterence]

If that can help in any way, I was able to run nixos-anywhere successfully on the free AMD 1GB oracle VM starting with Oracle Linux with some ram freeing commands beforehand:

ssh opc@_ip_
sudo -i
systemctl stop pmcd pmlogger pmproxy pmie
pkill -9 pmda
systemctl stop oracle-cloud-agent
systemctl stop oracle-cloud-agent-updater
pkill -u ocarun
systemctl stop firewalld
systemctl stop tuned
sync; echo 3 > /proc/sys/vm/drop_caches
echo 1 > /proc/sys/vm/compact_memory

Then nixos-anywhere --flake .#_machine_ --no-disko-deps opc@_ip_

I also had to make sure the networking config was correct else I wouldn't be able to SSH into the machine after reboot, here's my config:

boot = {
  loader.systemd-boot.enable = true;
  loader.efi.canTouchEfiVariables = false;

  initrd.availableKernelModules = [
    "virtio_pci"
    "virtio_scsi"
    "usbhid"
    "nvme"
  ];
};

networking.hostName = "free-amd-instance-1";
system.stateVersion = "25.11";

# User Configuration
users.users.nixos = {
  isNormalUser = true;
  extraGroups = [ "wheel" ];
  openssh.authorizedKeys.keys = [
    "ssh-ed25519 xxx"
  ];
};

users.users.root.openssh.authorizedKeys.keys = [
  "ssh-ed25519 xxx"
];

services.openssh = {
  enable = true;
  settings.PasswordAuthentication = false;
  settings.PermitRootLogin = "yes";
};

systemd.network.enable = false;
networking = {
  useDHCP = true;
  dhcpcd.enable = true;
  useNetworkd = false;
};
security.sudo.wheelNeedsPassword = false;
zramSwap = {
  enable = true;
  algorithm = "zstd";
  memoryPercent = 150; # 1GB RAM -> 1.5GB zram
  priority = 10;
};