Create SWAP Memory on Linux - CentOS, AlmaLinux, Ubuntu

SWAP memory is a crucial component of Linux virtual memory management that extends available system memory by using disk space as temporary storage for inactive processes. When physical RAM becomes full, the Linux kernel moves less frequently used data from RAM to SWAP space, preventing system crashes and maintaining performance.

This comprehensive guide covers creating, configuring, and optimizing SWAP memory on CentOS, AlmaLinux, and Ubuntu systems with best practices for 2020.

Understanding SWAP Memory

SWAP (also known as virtual memory) is a space on storage devices that the Linux kernel uses as an extension of physical RAM. When the system runs low on physical memory, the kernel moves inactive pages from RAM to SWAP space, freeing up RAM for active processes.

How SWAP Works

Component	Function	Performance Impact
Physical RAM	Fast access, primary memory	Fastest access speed
SWAP Space	Extended memory on disk	Slower than RAM, faster than regular file I/O
Kernel Memory Manager	Manages memory allocation	Automatic SWAP usage based on system load

Types of SWAP

1. SWAP Partition

Dedicated disk partition for SWAP
Better performance than SWAP files
Fixed size - requires repartitioning to resize
Recommended for production servers

2. SWAP File

Regular file used as SWAP space
Flexible sizing - can be resized without repartitioning
Slightly slower than SWAP partitions
Easier management for virtual machines and containers

When SWAP is Essential

Limited RAM: Systems with less than 4GB RAM
Hibernation Support: Required for suspend-to-disk functionality
Memory-intensive Applications: Databases, web servers, development environments
Virtual Machines: VPS and cloud instances with limited memory
Server Environments: Production servers requiring memory overflow protection

SWAP Sizing Recommendations

Proper SWAP sizing depends on system RAM, usage patterns, and specific requirements.

Traditional Sizing Guidelines

Physical RAM	Recommended SWAP Size	Use Case
< 2GB	2x RAM	Desktop systems, basic servers
2GB - 8GB	Equal to RAM	General purpose servers
8GB - 64GB	0.5x to 1x RAM	High-memory servers
> 64GB	4GB - 8GB minimum	Enterprise servers, databases

Modern Sizing Considerations (2020)

Cloud and VPS Environments

bash

# Recommended SWAP sizes for cloud instances
# 1GB RAM → 2GB SWAP
# 2GB RAM → 2GB SWAP  
# 4GB RAM → 2GB SWAP
# 8GB+ RAM → 1GB-2GB SWAP

Performance-Oriented Sizing

SSD Storage: Smaller SWAP sizes (1GB-2GB) due to faster I/O
HDD Storage: Larger SWAP sizes but consider performance impact
Memory-intensive Applications: Size based on application requirements

Hibernation Requirements

bash

# For hibernation support
SWAP_SIZE = RAM_SIZE + (RAM_SIZE * 0.1)
# Example: 8GB RAM = 8.8GB SWAP minimum

Prerequisites and System Check

Before creating SWAP, verify current system configuration and requirements.

Check Current SWAP Status

bash

# Check existing SWAP
swapon --show

# Alternative check
cat /proc/swaps

# Check memory usage
free -h

# Check disk space
df -h

System Requirements

CentOS/AlmaLinux Requirements

CentOS 7+ or AlmaLinux 8+
Root privileges for SWAP creation
Available disk space (recommended minimum 1GB)
ext4, xfs, or btrfs filesystem support

Ubuntu Requirements

Ubuntu 16.04+ (LTS recommended)
Root or sudo access
Available disk space for SWAP file
ext4 filesystem (default and recommended)

Check Available Disk Space

bash

# Check filesystem usage
df -h /

# Check available space in /var (common SWAP location)
df -h /var

# Check inode usage
df -i /

Creating SWAP on CentOS and AlmaLinux

Method 1: SWAP File Creation (Recommended)

Step 1: Create SWAP File

bash

# Create 2GB SWAP file using fallocate (faster)
sudo fallocate -l 2G /swapfile

# Alternative: Create using dd (more compatible)
sudo dd if=/dev/zero of=/swapfile bs=1M count=2048

# Verify file creation
ls -lh /swapfile

Step 2: Set Proper Permissions

bash

# Set restrictive permissions for security
sudo chmod 600 /swapfile

# Verify permissions
ls -l /swapfile
# Should show: -rw------- 1 root root

Step 3: Create SWAP Filesystem

bash

# Format file as SWAP
sudo mkswap /swapfile

# Expected output:
# Setting up swapspace version 1, size = 2 GiB (2147479552 bytes)
# no label, UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx

Step 4: Enable SWAP

bash

# Activate SWAP file
sudo swapon /swapfile

# Verify SWAP is active
swapon --show
free -h

Step 5: Make SWAP Permanent

bash

# Backup fstab
sudo cp /etc/fstab /etc/fstab.backup

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

# Verify fstab entry
tail -1 /etc/fstab

Method 2: SWAP Partition Creation

Step 1: Create Partition

bash

# List available disks
lsblk

# Create partition using fdisk
sudo fdisk /dev/sdb

# Follow these steps in fdisk:
# n (new partition)
# p (primary partition)
# 1 (partition number)
# Enter (default start)
# +2G (size)
# t (change type)
# 82 (Linux swap type)
# w (write changes)

Step 2: Format SWAP Partition

bash

# Format partition as SWAP
sudo mkswap /dev/sdb1

# Enable SWAP partition
sudo swapon /dev/sdb1

# Add to fstab for persistence
echo '/dev/sdb1 none swap sw 0 0' | sudo tee -a /etc/fstab

CentOS/AlmaLinux Specific Configuration

Configure Firewall (if needed)

bash

# SWAP doesn't require firewall rules, but ensure system tools work
sudo firewall-cmd --list-all

# If using monitoring tools, allow necessary ports
sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --reload

SELinux Considerations

bash

# Check SELinux status
getenforce

# If SELinux is enforcing, ensure proper context
sudo restorecon -v /swapfile

# Check SWAP file context
ls -Z /swapfile

Creating SWAP on Ubuntu

Method 1: SWAP File Creation (Ubuntu)

Step 1: Create SWAP File

bash

# Create 2GB SWAP file
sudo fallocate -l 2G /swapfile

# Alternative for older Ubuntu versions
sudo dd if=/dev/zero of=/swapfile bs=1M count=2048

# Verify creation
sudo ls -lh /swapfile

Step 2: Secure SWAP File

bash

# Set correct permissions
sudo chmod 600 /swapfile

# Verify permissions
ls -l /swapfile

Step 3: Setup SWAP

bash

# Make SWAP file
sudo mkswap /swapfile

# Enable SWAP
sudo swapon /swapfile

# Verify SWAP status
sudo swapon --show
free -h

Step 4: Persistent SWAP Configuration

bash

# Backup fstab
sudo cp /etc/fstab /etc/fstab.backup.$(date +%Y%m%d)

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

# Test fstab entry
sudo mount -a

Method 2: Using Ubuntu's Built-in Tools

Using Ubuntu's swap-management Tools

bash

# Install dphys-swapfile (if not present)
sudo apt update
sudo apt install dphys-swapfile

# Configure SWAP size
sudo nano /etc/dphys-swapfile

Edit the configuration:

bash

# Set SWAP size (in MB)
CONF_SWAPSIZE=2048

# Set SWAP file location
CONF_SWAPFILE=/var/swap

bash

# Apply configuration
sudo dphys-swapfile setup
sudo dphys-swapfile swapon

# Enable service
sudo systemctl enable dphys-swapfile

SWAP Performance Optimization

Configure Swappiness

Swappiness controls how aggressively the kernel swaps memory pages to disk.

Check Current Swappiness

bash

# View current swappiness value
cat /proc/sys/vm/swappiness
# Default is usually 60

Optimize Swappiness Values

Swappiness Value	Behavior	Use Case
0	Minimal swapping	High-performance servers
10	Reduce swapping	Database servers
30	Balanced approach	General servers
60	Default setting	Desktop systems
100	Aggressive swapping	Memory-constrained systems

Set Swappiness Temporarily

bash

# Set swappiness to 10 (current session)
sudo sysctl vm.swappiness=10

# Verify change
cat /proc/sys/vm/swappiness

Set Swappiness Permanently

bash

# Add to sysctl configuration
echo 'vm.swappiness=10' | sudo tee -a /etc/sysctl.conf

# Apply immediately
sudo sysctl -p

# Verify setting
sysctl vm.swappiness

Configure Cache Pressure

bash

# Optimize cache pressure (default is 100)
echo 'vm.vfs_cache_pressure=50' | sudo tee -a /etc/sysctl.conf

# Apply changes
sudo sysctl -p

Advanced Performance Tuning

Configure SWAP Behavior

bash

# Add advanced SWAP tuning to sysctl.conf
sudo tee -a /etc/sysctl.conf << EOF
# SWAP Performance Tuning
vm.swappiness=10
vm.vfs_cache_pressure=50
vm.dirty_background_ratio=5
vm.dirty_ratio=10
EOF

# Apply all settings
sudo sysctl -p

Monitor SWAP Performance

bash

# Real-time SWAP monitoring
watch -n 1 'free -h && echo "" && swapon -s'

# SWAP usage statistics
vmstat 1 5

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

Managing SWAP Memory

Monitor SWAP Usage

Basic Monitoring Commands

bash

# Quick SWAP status
free -h

# Detailed SWAP information
swapon -s

# SWAP usage by process
for file in /proc/*/status ; do awk '/VmSwap|Name/{printf $2 " " $3}END{ print ""}' $file; done | sort -k 2 -n | tail

# System memory summary
cat /proc/meminfo

Advanced Monitoring Script

bash

#!/bin/bash
# SWAP Monitoring Script

echo "=== SWAP Memory Status ==="
echo "Date: $(date)"
echo ""

echo "Current SWAP Usage:"
free -h | grep -E "(Mem|Swap)"
echo ""

echo "SWAP Devices:"
swapon --show
echo ""

echo "Top 10 Processes Using SWAP:"
for file in /proc/*/status ; do 
    awk '/VmSwap|Name/{printf $2 " " $3}END{ print ""}' $file 2>/dev/null
done | sort -k 2 -n | tail -10
echo ""

echo "Swappiness Setting:"
echo "vm.swappiness = $(cat /proc/sys/vm/swappiness)"

Resize SWAP File

Increase SWAP Size

bash

# Disable current SWAP
sudo swapoff /swapfile

# Create larger SWAP file
sudo fallocate -l 4G /swapfile

# Re-setup SWAP
sudo mkswap /swapfile
sudo swapon /swapfile

# Verify new size
swapon --show

Decrease SWAP Size

bash

# Disable SWAP
sudo swapoff /swapfile

# Create smaller SWAP file
sudo fallocate -l 1G /swapfile

# Setup new SWAP
sudo mkswap /swapfile
sudo swapon /swapfile

Remove SWAP

Temporary Removal

bash

# Disable SWAP
sudo swapoff /swapfile

# To re-enable
sudo swapon /swapfile

Permanent Removal

bash

# Disable SWAP
sudo swapoff /swapfile

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

# Delete SWAP file
sudo rm /swapfile

# Verify removal
swapon --show

Troubleshooting Common Issues

Issue 1: SWAP Not Activating

Symptoms: SWAP file created but not showing in swapon --show

Solutions:

bash

# Check file permissions
ls -l /swapfile
# Should be 600 (rw-------)

# Check if file is properly formatted
file /swapfile
# Should show: Linux/i386 swap file

# Re-format if necessary
sudo mkswap /swapfile
sudo swapon /swapfile

Issue 2: Performance Issues with SWAP

Symptoms: System becomes slow when SWAP is used

Solutions:

bash

# Check swappiness setting
cat /proc/sys/vm/swappiness

# Reduce swappiness for better performance
sudo sysctl vm.swappiness=10

# Check disk I/O performance
iostat -x 1 5

# Consider SSD storage for SWAP

Issue 3: SWAP File Creation Fails

Symptoms: fallocate or dd commands fail

Solutions:

bash

# Check available disk space
df -h /

# Check filesystem type
mount | grep " / "

# For filesystems that don't support fallocate
sudo dd if=/dev/zero of=/swapfile bs=1M count=2048

# Check for file system errors
sudo fsck /dev/sda1

Issue 4: Permission Denied Errors

Symptoms: Cannot create or access SWAP file

Solutions:

bash

# Check current user permissions
whoami
id

# Ensure running with sudo
sudo ls -l /swapfile

# Check SELinux context (CentOS/AlmaLinux)
ls -Z /swapfile
sudo restorecon -v /swapfile

Issue 5: SWAP Not Persistent After Reboot

Symptoms: SWAP disappears after system restart

Solutions:

bash

# Check fstab entry
grep swap /etc/fstab

# Add correct entry if missing
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

# Test fstab
sudo mount -a
sudo swapon -a

Best Practices and Security

Security Considerations

File Permissions

bash

# Always set restrictive permissions
sudo chmod 600 /swapfile
sudo chown root:root /swapfile

# Verify permissions
ls -l /swapfile

SWAP Encryption (Advanced)

bash

# For sensitive data, consider encrypted SWAP
sudo apt install cryptsetup  # Ubuntu
sudo yum install cryptsetup  # CentOS

# Setup encrypted SWAP (requires partition)
sudo cryptsetup luksFormat /dev/sdb1
sudo cryptsetup luksOpen /dev/sdb1 swap
sudo mkswap /dev/mapper/swap

Performance Best Practices

SSD Storage: Use SSD for SWAP when possible
Separate Disk: Place SWAP on different disk than root filesystem
Monitor Usage: Regular monitoring prevents performance issues
Optimize Swappiness: Tune based on application requirements
Regular Maintenance: Clean and optimize SWAP periodically

Monitoring and Maintenance

Create Monitoring Script

bash

#!/bin/bash
# /usr/local/bin/swap-monitor.sh

SWAP_THRESHOLD=80  # Alert when SWAP usage exceeds 80%

SWAP_USED=$(free | grep Swap | awk '{print $3}')
SWAP_TOTAL=$(free | grep Swap | awk '{print $2}')

if [ $SWAP_TOTAL -gt 0 ]; then
    SWAP_PERCENT=$(( SWAP_USED * 100 / SWAP_TOTAL ))
    
    if [ $SWAP_PERCENT -gt $SWAP_THRESHOLD ]; then
        echo "WARNING: SWAP usage is ${SWAP_PERCENT}%"
        echo "Consider investigating high memory usage processes"
        
        # Log to syslog
        logger "SWAP usage high: ${SWAP_PERCENT}%"
        
        # Optional: Send email alert
        # echo "SWAP usage is ${SWAP_PERCENT}%" | mail -s "High SWAP Usage Alert" [email protected]
    fi
fi

Schedule Regular Monitoring

bash

# Add to crontab for hourly checks
sudo crontab -e

# Add this line:
0 * * * * /usr/local/bin/swap-monitor.sh

Distribution-Specific Considerations

CentOS/AlmaLinux Specific

Package Requirements

bash

# Install required tools
sudo yum install util-linux procps-ng

# For CentOS 8/AlmaLinux
sudo dnf install util-linux procps-ng

Service Management

bash

# Check SWAP service status
sudo systemctl status dev-swapfile.swap

# Enable SWAP service
sudo systemctl enable dev-swapfile.swap

Ubuntu Specific

Package Requirements

bash

# Install monitoring tools
sudo apt update
sudo apt install sysstat htop

# For advanced SWAP management
sudo apt install dphys-swapfile

Ubuntu Cloud Images

bash

# Cloud images often have small SWAP, increase if needed
sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

Conclusion

SWAP memory is an essential component for stable Linux system operation, especially in environments with limited physical RAM. Proper SWAP configuration ensures system stability, prevents out-of-memory errors, and provides a safety net for memory-intensive applications.

Key Benefits of Proper SWAP Configuration

System Stability: Prevents crashes due to memory exhaustion
Performance Optimization: Allows better memory management
Application Support: Enables hibernation and large application support
Cost Effectiveness: Extends usable memory without hardware upgrades

Recommended SWAP Setup Summary

System Type	Recommended SWAP Size	Configuration
Desktop/Workstation	Equal to RAM	File-based SWAP
Server (< 8GB RAM)	1.5x to 2x RAM	Partition-based SWAP
Server (> 8GB RAM)	2GB to 4GB	File-based SWAP
Cloud/VPS	1GB to 2GB	File-based SWAP

Performance Impact

Well-configured SWAP provides:

Minimal Performance Impact: When properly sized and tuned
System Resilience: Protection against memory exhaustion
Better Resource Utilization: Optimal use of available resources
Application Stability: Support for memory-intensive workloads

Regular monitoring, proper sizing, and performance tuning ensure that SWAP enhances rather than hinders system performance. Whether running CentOS, AlmaLinux, or Ubuntu, following these guidelines will provide robust virtual memory management for your Linux systems.

Remember to monitor SWAP usage regularly and adjust configuration based on actual system requirements and usage patterns. With proper implementation, SWAP memory becomes a valuable tool for maintaining system performance and stability.