Linux Swap Memory and File - Virtual Memory Guide

Check current swap usage and configuration.

# Check memory and swap
free -h

# Show swap spaces
swapon --show
cat /proc/swaps

# Detailed memory info
cat /proc/meminfo | grep -i swap

# Real-time monitoring
watch -n 1 'free -h'

# Show swap usage per process
for file in /proc/*/status; do 
    awk '/VmSwap|Name/{printf $2 " " $3}END{ print ""}' $file 2>/dev/null
done | sort -k2 -n -r | head

# Container memory stats
docker stats --no-stream
kubectl top pods

Create and manage swap files dynamically.

# Create 4GB swap file
sudo dd if=/dev/zero of=/swapfile bs=1G count=4

# Set secure permissions
sudo chmod 600 /swapfile

# Format as swap
sudo mkswap /swapfile

# Enable swap
sudo swapon /swapfile

# Disable swap
sudo swapoff /swapfile

# Permanent setup - add to /etc/fstab
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

# Remove swap file
sudo swapoff /swapfile
sudo rm /swapfile

# Resize swap file
sudo swapoff /swapfile
sudo dd if=/dev/zero of=/swapfile bs=1G count=8
sudo mkswap /swapfile
sudo swapon /swapfile

Optimize swap behavior for better performance.

# Check current settings
sysctl vm.swappiness
sysctl vm.vfs_cache_pressure

# Set swappiness (0-100)
# Lower = less swap usage
sudo sysctl vm.swappiness=10

# Set vfs cache pressure
sudo sysctl vm.vfs_cache_pressure=50

# Make changes permanent
echo 'vm.swappiness=10' | sudo tee -a /etc/sysctl.conf
echo 'vm.vfs_cache_pressure=50' | sudo tee -a /etc/sysctl.conf

# Apply changes
sudo sysctl -p

# Check dirty pages (write cache)
sysctl vm.dirty_ratio
sysctl vm.dirty_background_ratio

# For database servers
sudo sysctl vm.swappiness=1
sudo sysctl vm.dirty_ratio=15
sudo sysctl vm.dirty_background_ratio=5

Manage swap for Docker containers and hosts.

# Run container with memory limits
docker run -it --memory=512m --memory-swap=1g ubuntu

# Set swap equal to memory (no extra swap)
docker run -it --memory=512m --memory-swap=512m ubuntu

# Disable swap completely
docker run -it --memory=512m --memory-swap=-1 ubuntu

# Global Docker daemon settings
# /etc/docker/daemon.json
{
  "memory-swap": -1,
  "memory": "1g"
}

# Check container memory usage
docker stats

# Set swap for running container
docker update --memory-swap 2g container_name

# Compose file example
# docker-compose.yml
services:
  app:
    deploy:
      resources:
        limits:
          memory: 512M
          memory-swap: 1G

Configure memory and swap in Kubernetes.

# Pod with memory limits
apiVersion: v1
kind: Pod
metadata:
  name: memory-demo
spec:
  containers:
  - name: memory-demo-ctr
    image: polinux/stress
    resources:
      limits:
        memory: "200Mi"
      requests:
        memory: "100Mi"

# Disable swap on kubelet
# /etc/default/kubelet
KUBELET_EXTRA_ARGS="--fail-swap-on=false"

# Or disable system swap
sudo swapoff -a

# ResourceQuota for namespace
apiVersion: v1
kind: ResourceQuota
metadata:
  name: mem-swap-demo
spec:
  hard:
    requests.memory: 1Gi
    limits.memory: 2Gi

# Check resource usage
kubectl top pods
kubectl describe nodes | grep -A10 "Allocated resources"

Monitor swap usage and set up alerts.

# node_exporter metrics
node_memory_SwapTotal_bytes
node_memory_SwapFree_bytes
node_memory_SwapCached_bytes

# Prometheus alert rules
groups:
- name: memory.rules
  rules:
  - alert: HighSwapUsage
    expr: (1 - (node_memory_SwapFree_bytes / node_memory_SwapTotal_bytes)) * 100 > 80
    for: 5m
    labels:
      severity: warning
    annotations:
      summary: "High swap usage detected"

# Custom monitoring script
#!/bin/bash
SWAP_USAGE=$(free | awk 'NR==3{printf "%.2f", $3/$2*100}')
if (( $(echo "$SWAP_USAGE > 80" | bc -l) )); then
    echo "ALERT: High swap usage: ${SWAP_USAGE}%"
    # Send to Slack/Email/PagerDuty
fi

# Container memory monitoring
docker stats --format "table {{.Name}}\t{{.MemUsage}}\t{{.MemPerc}}"