ZRAM in NixOS - download more RAM?

This guide will show how to enable and set up ZRAM on a normal desktop machine (PC or laptop). Servers, smart fridges, stupid TVs, and your grandma’s wrist watch might need a different setup.

What is ZRAM?

ZRAM is a technology that compresses data in RAM. It lets you create a virtual block of memory that works like normal swap, but way faster and without using disk space.

In NixOS, ZRAM is super easy to set up through the system configuration.nix - literally two minutes and you’re done.

Why use ZRAM?

1. Saves disk space - no need to reserve swap space.
2. Speed - ZRAM is much faster than standard swap, especially with the right compression algorithm.
3. Reduces SSD wear - self explanatory.

ZRAM quirks

1. If you have a laptop and want hibernation, you still need a swap partition the size of your RAM.
2. Can use more CPU for compressing/decompressing data. But with the right algorithm, it’s mostly negligible.

Setting up ZRAM on NixOS

Step 1: Enable ZRAM

{
  zramSwap = {
    enable = true;
  }
}

…but that would be a shitty guide if I didn’t explain how to actually tune it for your system.

Step 2: Set priority

Set a priority so the system uses ZRAM when swap is needed. You can skip this, but I strongly recommend specifying it:

{
  zramSwap = {
    ...
    priority = 100; # example value
  }
}

• Default swap priority is usually -2. You can check that with swapon --show command.
• Priority set to 100 is usually enough; no need to go higher unless you have a good reason.

Step 3: Pick an algorithm

Algorithms are really important if you want yourself the best ZRAM config. Here’s what NixOS offers:

1. zstd (Zstandard) - as the name implies it’s a standard algorithm. Good compression, but uses CPU more than other algorithms do. The price to pay is small, but can matter on low-powered machines.
2. lz4 - compresses less than zstd, but a lot faster and uses less CPU.
3. lzo - compresses even less than lz4, very fast, minimal CPU usage.
4. lz4hc - compresses more than lz4 but less than zstd, faster than zstd, slower than lz4, more CPU usage than lz4.

Other algorithms exist and are available in configuration.nix, but these four are enough for any machine.

“Which algorithm should I pick?! I don’t get it”

• lz4. In 99% of cases, it’s fine. Fast, light, still compresses well.

Enable an algorithm:

{
  zramSwap = {
    ...
    algorithm = "lz4"; # example, pick what you need
  }
}

Step 4: Set memoryPercent

memoryPercent sets the MAX amount of RAM ZRAM can use. Important: ZRAM does not steal your RAM. It only compresses memory when swap is actually needed.

• Example: 16GB RAM, memoryPercent = 50 → it won’t grab 8GB right away. It just means ZRAM can compress up to 8GB if swap is needed.

Simple examples:

• Need 300MB swap → ZRAM (lz4) compresses around 150MB RAM
• Need 7GB swap → ZRAM compresses ~3.5GB RAM
• Need 20GB swap → ZRAM tries 10GB, but limit is 8GB → only 8GB compressed, rest is like swap ran out

When idle, ZRAM does nothing - your RAM is fully yours.

Set it in config:

{
  zramSwap = {
    ...
    memoryPercent = 50; # example value
  }
}

Step 5: Make sure that everything is good

In the end your ZRAM config should look something like this:

{
  zramSwap = {
    enable = true;
    priority = 100;
    algorithm = "lz4";
    memoryPercent = 50;
  };
}

This configuration is solid and should work just fine. This is what I rock on my main machine.

Final steps

After declaring ZRAM in your config:

sudo nixos-rebuild switch

…Or whatever, you know how to rebuild…

Reboot. It’s needed, frfr.

Then you cab check ZRAM status and make sure that all is in fact good:

swapon --show

That’s it!

Thanks everyone! Love your mom, and use ZRAM!