zram-swap/README.md

zram-swap

A simple zram swap script for modern systemd Linux

https://github.com/foundObjects/zram-swap

Why?

I wrote zram-swap because I couldn't find a simple modern replacement for the Ubuntu zram-config package that included basic error handling, didn't make device sizing mistakes and kept user-facing configuration straightforward and easy to understand.

Installation and Usage

Arch Linux:

Install from the AUR: paru -S zram-swap-git

or directly from GitHub:

mkdir zram-swap-git && cd zram-swap-git
wget https://raw.githubusercontent.com/foundObjects/zram-swap/arch-packaging/PKGBUILD
makepkg -Cci

You'll need to start and enable zram-swap.service after installation on Arch. Make any configuration changes to /etc/default/zram-swap first if desired, see below for details.

Others:

git clone https://github.com/foundObjects/zram-swap.git
cd zram-swap && sudo ./install.sh

The install script starts the zram-swap.service automatically after installation and enables the systemd service during boot. The default allocation creates an lz4 zram device that should use around half of physical memory when completely full.

I chose lz4 as the default to give low spec machines (systems that often see the greatest benefit from swap on zram) every performance edge I could. While lzo-rle is quite fast on modern high-performance hardware a machine like a Raspberry Pi or a low spec laptop appreciates every speed advantage I can give it.

Configuration

Edit /etc/default/zram-swap if you'd like to change the compression algorithm or swap allocation and then restart zram-swap with systemctl restart zram-swap.service. The configuration file is heavily commented and self-documenting.

A very simple configuration that's expected to use roughly 2GB RAM might look something like:

# override fractional calculations and specify a fixed swap size
_zram_fixedsize="6G"

# compression algorithm to employ (lzo, lz4, zstd, lzo-rle)
_zram_algorithm="lzo-rle"

Remember that the ZRAM device size references uncompressed data, real memory utilization should be ~2-3x smaller than the zram device size due to compression.

A quick note RE: compression algorithms:

The default configuration using lz4 should work well for most people. lzo may provide slightly better RAM utilization at a cost of slightly more expensive decompression. zstd should provide better compression than lz* and still be moderately fast on most machines. On very modern kernels and reasonably fast hardware the most balanced choice is probably lzo-rle. On low spec machines (ARM SBCs, ARM laptops, thin clients, etc) you'll probably want to stick with lz4.

Debugging

To view a script debug trace either start zram-swap.sh with zram-swap.sh -x (start|stop) or uncomment the debug flag in /etc/default/zram-swap:

# setting _zram_swap_debugging to any non-zero value enables debugging
# default: undefined
_zram_swap_debugging="beep boop"

Compatibility

Tested on Linux 4.4 through Linux 5.14.

Requirements are minimal; Underneath the systemd service wrapper the swap setup script needs only a posix shell, modprobe, zramctl and very basic awk and grep support to function. It should work in pretty much any modern Linux environment.