Zram: Compressed Swap in Ram

zram creates a compressed block device that lives entirely in RAM and is typically used as swap. Because it's RAM-backed and compressed, swapping to it is far faster than swapping to disk — it buys extra headroom under memory pressure without the latency hit of a real disk-backed swap file (see the swap-file lesson in this track for the disk-backed alternative).

Install

Ubuntu/Debian uses zram-tools:

sudo apt install zram-tools
sudo systemctl enable --now zramswap.service

Fedora uses systemd-zram-generator instead:

sudo dnf install zram-generator

Fedora may already ship a default zram config — creating a config file overrides those defaults.

Configure

Ubuntu/Debian (/etc/default/zramswap):

sudo nvim /etc/default/zramswap

# Compression algorithm
# speed:        lz4 > zstd > lzo
# compression:  zstd > lzo > lz4
# available algorithms: /sys/block/zram0/comp_algorithm
ALGO=zstd

# Percentage of total RAM to use for zram
PERCENT=75

# Fixed size in MiB (ignored when PERCENT is set)
#SIZE=256

# Swap priority (higher = preferred first)
PRIORITY=100

sudo systemctl restart zramswap.service

Fedora (/etc/systemd/zram-generator.conf):

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

[zram0]
zram-size = min(ram, 12288)

This uses zram up to the size of RAM, capped at 12 GB, with the kernel's default compression algorithm — specifying one explicitly is optional; the default is stable and well-tested.

sudo systemctl daemon-reexec
sudo reboot

Verify

swapon --show                         # confirm zram swap is active
zramctl                                # inspect zram devices
cat /sys/block/zram0/comp_algorithm    # check the active algorithm

Note

• On Fedora, no manual service start is needed after reboot — the systemd generator sets zram up early at boot.
• For most desktop workloads, the kernel's default compression algorithm is more than sufficient; there's rarely a need to tune it further.
• Priority: Always ensure zram has a higher priority (e.g., PRIORITY=100) than traditional disk swap files/partitions, so compressed RAM is consumed before writing to SSD/HDD storage.

References: kernel.org zram docs, systemd/zram-generator