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

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

Start zram-swap.sh with zram-swap.sh -x (start|stop) to view the debug trace and determine what's going wrong.

To dump the full execution trace during service start/stop edit /etc/systemd/systemd/zram-swap.service and add -x to the following two lines:

ExecStart=/usr/local/sbin/zram-swap.sh -x start
ExecStop=/usr/local/sbin/zram-swap.sh -x stop

Compatibility

Tested on Linux 4.4 through Linux 5.10.

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.