Swap Configuration

When to Use Swap

RAM Size	Swap Size	Recommendation
< 2GB	2 × RAM	Essential for low-memory servers
2-4GB	1 × RAM	Good safety net
4-8GB	4GB	Adequate
8-16GB	2-4GB	Minimal
> 16GB	2GB	Rarely needed

Check Current Swap

# Check if swap is active
swapon --show

# Check memory & swap usage
free -h

# Check filesystem for swap file
ls -lh /swapfile

Create Swap File

# 1. Create swap file (4GB example)
fallocate -l 4G /swapfile
# OR (if fallocate not available)
dd if=/dev/zero of=/swapfile bs=1M count=4096

# 2. Set correct permissions
chmod 600 /swapfile

# 3. Format as swap
mkswap /swapfile

# 4. Enable swap
swapon /swapfile

# 5. Verify
swapon --show
free -h

Make Swap Permanent

# Add to /etc/fstab
echo '/swapfile none swap sw 0 0' | tee -a /etc/fstab

# Verify fstab entry
cat /etc/fstab | grep swap

Swap Swappiness

Swappiness controls how aggressively the kernel uses swap (0-100).

# Check current value
cat /proc/sys/vm/swappiness

# Recommended values:
# 10 — Good for servers (less swap usage)
# 60 — Default (balance)
# 100 — Aggressive swap

# Set temporarily
sysctl vm.swappiness=10

# Make permanent
echo 'vm.swappiness=10' | tee -a /etc/sysctl.conf

Remove Swap

# 1. Disable swap
swapoff /swapfile

# 2. Remove from fstab
sed -i '/swapfile/d' /etc/fstab

# 3. Delete file
rm /swapfile

# 4. Verify
swapon --show
free -h

Swap for Docker

Docker containers can also use swap. Set limits:

docker-compose.yml

services:
  app:
    # Memory + swap limit
    mem_limit: 512m
    memswap_limit: 1g  # 512MB RAM + 512MB swap

Monitoring Swap Usage

# Real-time swap usage
free -h -s 5

# Processes using swap
for pid in /proc/*/status; do
    awk '/VmSwap|Name/{printf "%s ", $2} END{print ""}' $pid 2>/dev/null
done | sort -k2 -n -r | head -10

# Swap usage per process
top -o %MEM
# Press 'c' for command, check SWAP column

# Or use smem
apt install -y smem
smem -s swap -r

Swap on SSD vs HDD

Drive Type	Notes
SSD	Fast swap, but wear from writes. Reduce swappiness to 10.
HDD	Slow swap, avoid if possible. Add more RAM instead.
NVMe	Excellent swap performance. Still reduce swappiness.

Verdict

# Quick check script
echo "=== Swap Status ==="
swapon --show
echo ""
echo "=== Memory ==="
free -h
echo ""
echo "=== Swappiness ==="
cat /proc/sys/vm/swappiness
echo ""
echo "=== Swap Usage by Top Processes ==="
for pid in /proc/*/status; do
    awk '/VmSwap|Name/{printf "%s ", $2} END{print ""}' $pid 2>/dev/null
done | sort -k2 -n -r | head -5