zram-swap

A simple zram swap script and service for modern systemd Linux

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

Why?

There are dozens of zram swap scripts; Unfortunately most are a bit crufty: they generally don't handle errors well, or they implement obsolete performance hacks from Linux 3.14 and almost all of them include logic errors in their device size calculations.

I wrote zram-swap because I couldn't find a replacement for the Ubuntu zram-config package that was simple, handled failures well, didn't make mistakes with device sizes and kept its user-facing configuration straightforward and easy to use.

Installation

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

Usage

The installer will start the zram-swap.service automatically after installation and enable service start during each subsequent boot. The default allocation creates an lz4 zram device that should use around half of physical memory when completely full.

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 best overall choice is probably lzo-rle.

I chose lz4 as the default to give low spec machines (some of the greatest beneficiaries of swap on zram) whatever performance edge I could. While lzo-rle is quite fast on modern hardware a machine like a Raspberry Pi 2B needs every optimization advantage I can give it.

Edit /etc/default/zram-swap if you'd like to change compression algorithms or swap allocation and then restart zram-swap with systemctl restart zram-swap.service.

Run zramctl during use to monitor swap compression and real memory usage.

A very simple configuration that's expected to use up to 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 device size due to compression.

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 5.7.

This should run on pretty much any recent (4.0+? kernel) Linux system using systemd. If anyone wants to try it on something really old and let me know how compatible the script is with older systems I'm definitely interested, but I don't have the legacy systems or time to test exhaustively.

The script itself should also be fully compatible with SysVinit compatible init systems. PRs making improvement there are welcome!