zram Configuration Guide

zram creates a compressed swap space in RAM. It is a good replacement of swap partition or swap file.

Preparation

Check the existing swap spaces

cat /proc/swaps

We will want to disable the existing swap spaces.

Disable zswap

Check if zswap is enabled:

cat /sys/module/zswap/parameters/enabled

If N is returned, then zswap is not enabled.

To disable zswap permanently on kernels where it is enabled by default, assuming that GRUB is the boot loader, edit /etc/default/grub:

sudo nano /etc/default/grub

You will find the GRUB_CMDLINE_LINUX_DEFAULT line, which may look similar to this:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

Append zswap.enabled=0 to the options, save and exit, then re-generate the grub-cfg file with:

sudo grub-mkconfig -o /boot/grub/grub.cfg

Disable swap partition

Edit /etc/fstab and remove the entry similar to:

UUID=<device_UUID> none swap defaults 0 0

Disable swap file

Locate the swap file as described at the beginning of this section. Suppose that the swap file is located at /swapfile, turn it off:

sudo swapoff /swapfile

Remove the swap file, be extra careful for the file name:

sudo rm -f /swapfile

Edit /etc/fstab and remove the entry similar to:

/swapfile none swap sw 0 0

Disable swap file on Raspberry Pi OS

Turn off the swap file:

sudo dphys-swapfile swapoff

Remove the swap file:

sudo dphys-swapfile uninstall

Disable the relevant systemd service:

sudo systemctl disable dphys-swapfile.service

Reboot

Reboot after any changes.

Configuring zram

Using zram-tools

Install:

sudo apt install zram-tools

Configure:

sudo nano /etc/default/zramswap

With the following configuration:

# Use the zstd compression algorithm
ALGO=zstd
# Scale zram to full memory size
PERCENT=100

Save and exit, then restart the zram service:

sudo systemctl restart zramswap

Using zram-generator

Install:

# For Arch Linux
sudo pacman -S zram-generator
# For Ubuntu
sudo apt install systemd-zram-generator

Configure:

sudo nano /etc/systemd/zram-generator.conf

With the following configuration (ref: https://man.archlinux.org/man/zram-generator.conf.5):

[zram0]
# Scale zram to full memory size
zram-size = ram
# Use the zstd compression algorithm
compression-algorithm = zstd

Reload the systemd daemon

sudo systemctl daemon-reload

Start the zram service:

sudo systemctl start systemd-zram-setup@zram0.service

Note on choosing the compression algorithm

speed: lz4 > zstd > lzo

compression: zstd > lzo > lz4

Note on choosing the zram size

Ref: https://fedoraproject.org/wiki/Changes/Scale_ZRAM_to_full_memory_size

The size of zram is the maximum uncompressed data size within the zram, hence it can be set to be equal to or even greater than the system’s physical memory size, as long as the compressed size does not exceed the physical memory capacity.

My own rationale for scaling the zram to the full physical memory size is that it is both not too aggressive, as this should not overflow the memory since the compressed zram should be smaller than the physical memory, while also not too conservative, as more data can be stored in the zram than the default values. However, this needs to stand the test of time.

For a more conservative configuration, set PERCENT=50 for zram-tools, or zram-size = min(ram / 2, 4096) for zram-generator.

Tuning zram

Configure swap behavior

Ref: https://wiki.archlinux.org/title/Zram#Optimizing_swap_on_zram

Create the following file:

sudo nano /etc/sysctl.d/99-vm-zram-parameters.conf

With the following content:

vm.swappiness = 180
vm.watermark_boost_factor = 0
vm.watermark_scale_factor = 125
vm.page-cluster = 0

Reboot

Reboot after any changes.

References

https://wiki.archlinux.org/title/Zram

https://wiki.archlinux.org/title/Zswap#Toggling_zswap

https://wiki.archlinux.org/title/Swap

https://forums.raspberrypi.com/viewtopic.php?t=244130

https://github.com/garyexplains/examples/blob/master/how_to_enable_ZRAM_Raspberry_Pi.md