q40-swap-configuration

Énoncé§

Solve this question on: data-002

1. Add a 1G swap file at /swapfile, persistent across reboots.
2. Add a swap partition on /dev/vdc1 with priority=10, also persistent.
3. Set vm.swappiness to 20.
4. Write the total amount of swap (in MB) into /opt/course/40/swap-mb.

Solution§

Step 1 — Swap file§

sudo fallocate -l 1G /swapfile          # fast, sparse-ish
# or, more portable:
sudo dd if=/dev/zero of=/swapfile bs=1M count=1024 status=progress

sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile
swapon --show

Persist in /etc/fstab:

/swapfile  none  swap  sw  0 0

Step 2 — Swap partition§

Create the partition (use fdisk, parted, or cfdisk; type 82 = Linux swap on MBR, 19 in parted):

sudo fdisk /dev/vdc        # n -> partition, t -> type 82, w -> write
sudo partprobe /dev/vdc

Format + activate with a priority:

sudo mkswap -L bigswap /dev/vdc1
sudo swapon --priority 10 /dev/vdc1
swapon --show
# NAME       TYPE      SIZE USED PRIO
# /dev/vdc1  partition   2G   0B   10
# /swapfile  file        1G   0B   -2

Higher priority is used first. With equal priorities, swap is striped (parallel).

Persist (use UUID for stability):

sudo blkid /dev/vdc1
echo 'UUID=...  none  swap  sw,pri=10  0 0' | sudo tee -a /etc/fstab

Step 3 — Swappiness§

sudo sysctl -w vm.swappiness=20

# persistent: see [[q25-sysctl-kernel-parameters]]
echo "vm.swappiness = 20" | sudo tee /etc/sysctl.d/99-swap.conf
sudo sysctl --system

• 0 → swap only to avoid OOM
• 60 → distro default on many systems
• 100 → swap as eagerly as possible

Companion: vm.vfs_cache_pressure (default 100) controls reclaim of inode/dentry cache.

Step 4 — Total swap in MB§

free -m | awk '/Swap:/ {print $2}' > /opt/course/40/swap-mb
# alternative:
awk '/SwapTotal/ {print $2/1024}' /proc/meminfo

Day-to-day commands§

swapon --show                          # active swaps
free -h                                # mem + swap
cat /proc/swaps
sudo swapoff /swapfile                 # deactivate
sudo swapoff -a                        # all
sudo swapon -a                         # everything in fstab

Memory pressure observation§

vmstat 1                               # si / so columns = swap in/out per sec
sar -W 1                               # paging stats
top                                    # press 'M' to sort by mem

zram (RAM-compressed swap) — alternative§

Useful on RAM-constrained systems:

sudo apt install zram-tools
sudo systemctl enable --now zramswap
zramctl

Sizing rule of thumb§

Modern Linux Foundation guidance (LFCS-ish):

• ≤ 2 GB RAM → swap = 2× RAM
• 2–8 GB RAM → swap = RAM
• 8–64 GB RAM → swap = RAM / 2 (min 4 GB)
• 64 GB RAM → 4 GB swap suffices

If hibernation is required, swap must be at least RAM size.