hp-chell/ZRAM.md

zram swap on hp-chell (KDE neon)

Set up 2026-03-05, reconstructed on 2026-09-27 from ~/.bash_history and the live config. It replaces the disk swapfile with compressed swap in RAM. That suits this machine: 15GiB RAM, and neon's root lives on a slow external USB/SD card (sda) that you don't want to swap to or wear out.

Current state (2026-09-27)

$ zramctl
NAME       ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 zstd            8G   4K   59B   20K         [SWAP]
$ swapon --show
NAME       TYPE      SIZE USED PRIO
/dev/zram0 partition   8G   0B  100

The zram device is the only swap. zswap is off (/sys/module/zswap/parameters/enabled = N), and there's nothing zram-related on the kernel command line.

What was done

1. Installed zram-tools (Debian's simple zramswap service; not systemd-zram-generator, which isn't installed): sudo apt install zram-tools, sudo systemctl enable --now zramswap.

2. Configured /etc/default/zramswap. The file is still the package's own conffile (dpkg -V zram-tools flags it as modified), with these lines added at the end:

   ENABLED=true
   ALGO=zstd
   SIZE=8192
   PRIORITY=100
   

   • ALGO=zstd: better compression than the default lz4, at some CPU cost. Fine on this 4-core Core m7 for swap-sized traffic.
   • SIZE=8192: an 8GiB zram device, a fixed size in MiB. PERCENT isn't set; if set, it would override SIZE. That's the uncompressed capacity; actual RAM used is the compressed size (COMPR/TOTAL in zramctl).
   • PRIORITY=100: above any disk swap, so zram is used first.

   Tuned over several stop/edit/start rounds (the history shows systemctl stop zramswap / nano / start three or four times). Only the final values are known. Apply changes with sudo systemctl restart zramswap.

3. Disabled the disk swapfile: in /etc/fstab,

   # removed swapfile, using zram instead
   #/swapfile                                 swap           swap    defaults   0 0
   

   then sudo swapoff /swapfile.

4. Raised vm.swappiness to 100, at runtime only (sudo sysctl vm.swappiness=100). See the loose ends below.

Loose ends (fixed 2026-09-27)

• vm.swappiness=100 hadn't been made persistent: it had been set with sysctl at runtime only and was back to the default 60. Now persisted in /etc/sysctl.d/99-zram.conf (vm.swappiness=100) and applied live. Swapping to zram is cheap, so a high swappiness is the usual choice. Not changed: vm.page-cluster (still 3). Some zram guides set it to 0, since zram has no seek cost and readahead just wastes work.
• The unused /swapfile (512MiB, dated 2026-03-04, fstab line commented out) was deleted.

Reapplying on a fresh neon install

sudo apt install zram-tools
sudo tee -a /etc/default/zramswap >/dev/null <<'EOF'
ENABLED=true
ALGO=zstd
SIZE=8192
PRIORITY=100
EOF
sudo systemctl enable --now zramswap
# comment out any /swapfile line in /etc/fstab, then:
sudo swapoff /swapfile
# persist swappiness
echo 'vm.swappiness=100' | sudo tee /etc/sysctl.d/99-zram.conf && sudo sysctl --system

Check with zramctl and swapon --show.