🔄 AlmaLinux vs CentOS: Complete Migration Guide for Beginners

Worried about your CentOS future? Don’t be! 🎯 Today we’ll explore everything about migrating from CentOS to AlmaLinux - the community-driven, enterprise-grade Linux that’s taking the world by storm! Whether you’re running one server or thousands, this guide makes migration simple and stress-free! 🚀

🤔 Why is AlmaLinux the Best CentOS Alternative?

Choosing AlmaLinux brings incredible advantages:

• 📌 1:1 RHEL compatibility - Drop-in replacement for CentOS with binary compatibility
• 🔧 Long-term support - 10 years of updates and security patches guaranteed
• 🚀 Community-driven - No single company control, truly open source
• 🔐 Enterprise-ready - Production-grade stability trusted by Fortune 500 companies
• ⭐ Free forever - No licensing fees, no subscription requirements

🎯 What You Need

Before starting your migration journey:

• ✅ Current CentOS 7, 8, or Stream system
• ✅ Root or sudo access to your server
• ✅ Backup of critical data (always backup first!)
• ✅ 30-60 minutes for migration (depending on system size)
• ✅ Internet connection for downloading packages

📝 Step 1: Understanding the Key Differences

Let’s understand what makes AlmaLinux special! 📊

CentOS vs AlmaLinux Comparison

# Check your current CentOS version
cat /etc/centos-release
# Example output: CentOS Linux release 8.5.2111

# Key differences to understand:
echo "=== CentOS vs AlmaLinux Comparison ==="
echo "CentOS 8: EOL December 31, 2021 (already ended!)"
echo "CentOS 7: EOL June 30, 2024 (ending soon!)"
echo "CentOS Stream: Rolling release (not stable for production)"
echo ""
echo "AlmaLinux 8: Supported until 2029"
echo "AlmaLinux 9: Supported until 2032"
echo "AlmaLinux: Stable releases perfect for production!"

Why Organizations Are Switching

# Real-world migration statistics
echo "📊 Migration Facts:"
echo "- 80% of CentOS users considering AlmaLinux"
echo "- 500,000+ AlmaLinux installations worldwide"
echo "- Major companies already migrated: CloudLinux, cPanel, Plesk"
echo "- Zero licensing costs vs RHEL subscription fees"

Pro tip: 💡 AlmaLinux is backed by the AlmaLinux OS Foundation, a 501(c)(6) non-profit, ensuring it stays free and open forever!

🔧 Step 2: Pre-Migration Preparation

Prepare your system for a smooth migration:

Backup Your System

# Create a full system backup (adjust paths as needed)
echo "🔒 Creating system backup..."

# Backup important configurations
sudo tar -czf /backup/etc-backup-$(date +%Y%m%d).tar.gz /etc/
sudo tar -czf /backup/home-backup-$(date +%Y%m%d).tar.gz /home/

# Backup database if you have one
if command -v mysqldump &> /dev/null; then
    mysqldump --all-databases > /backup/mysql-backup-$(date +%Y%m%d).sql
fi

# List installed packages for reference
rpm -qa > /backup/installed-packages-$(date +%Y%m%d).txt

echo "✅ Backup completed! Files saved in /backup/"

System Health Check

# Check system health before migration
echo "🏥 Performing system health check..."

# Check disk space (need at least 5GB free)
df -h / | grep -v Filesystem

# Check current kernel
uname -r

# Check for any broken packages
sudo rpm -Va | grep -E '^..5'

# Update current system
sudo yum update -y

echo "✅ System ready for migration!"

Document Current Configuration

# Save current system state for comparison
echo "📋 Documenting system configuration..."

# Network configuration
ip addr show > /backup/network-config.txt

# Service status
systemctl list-units --state=running > /backup/running-services.txt

# Firewall rules
sudo firewall-cmd --list-all > /backup/firewall-rules.txt

# Repository list
yum repolist > /backup/repo-list.txt

echo "✅ Configuration documented!"

🌟 Step 3: Performing the Migration

Now for the exciting part - the actual migration! 🎉

Method 1: Using AlmaLinux Migration Tool (Recommended)

# Download and run the official migration script
echo "🚀 Starting AlmaLinux migration..."

# Download the migration tool
curl -O https://raw.githubusercontent.com/AlmaLinux/almalinux-deploy/master/almalinux-deploy.sh

# Make it executable
chmod +x almalinux-deploy.sh

# Run the migration (this will take 15-30 minutes)
sudo ./almalinux-deploy.sh

# The script will:
# 1. Check system compatibility
# 2. Replace CentOS repositories with AlmaLinux
# 3. Update all packages to AlmaLinux versions
# 4. Update boot loader and kernel

Method 2: Manual Migration for CentOS 8

# Manual migration steps for more control
echo "🔧 Manual migration process..."

# Step 1: Remove CentOS branding packages
sudo rpm -e --nodeps centos-linux-release centos-linux-repos centos-gpg-keys centos-logos

# Step 2: Install AlmaLinux release packages
sudo rpm -Uvh https://repo.almalinux.org/almalinux/8/BaseOS/x86_64/os/Packages/almalinux-release-8.9-1.el8.x86_64.rpm
sudo rpm -Uvh https://repo.almalinux.org/almalinux/8/BaseOS/x86_64/os/Packages/almalinux-repos-8.9-1.el8.x86_64.rpm
sudo rpm -Uvh https://repo.almalinux.org/almalinux/8/BaseOS/x86_64/os/Packages/almalinux-gpg-keys-8.9-1.el8.x86_64.rpm

# Step 3: Clean and update
sudo dnf clean all
sudo dnf distro-sync -y

# Step 4: Install AlmaLinux kernel
sudo dnf install -y kernel kernel-core kernel-modules

# Step 5: Update GRUB
sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Monitoring Migration Progress

# Watch migration progress in another terminal
watch -n 2 'tail -20 /var/log/dnf.log'

# Check for any errors
grep -i error /var/log/dnf.log

# Monitor system resources during migration
top

What happens during migration: 🔄

• Repository URLs are updated to AlmaLinux mirrors
• All packages are synchronized with AlmaLinux versions
• System branding is updated to AlmaLinux
• Kernel and bootloader are updated
• All your data and configurations remain intact!

✅ Step 4: Post-Migration Verification

Verify your successful migration:

System Verification

# Verify AlmaLinux installation
echo "🔍 Verifying migration success..."

# Check OS version
cat /etc/redhat-release
# Should show: AlmaLinux release 8.9 (Midnight Oncilla)

# Verify kernel
uname -r | grep el8

# Check AlmaLinux repos
dnf repolist | grep -i alma

# Verify system is up to date
sudo dnf check-update

echo "✅ Migration verified successfully!"

Service Verification

# Check all services are running properly
echo "🔧 Checking services..."

# Compare with pre-migration service list
systemctl list-units --state=running > /tmp/new-services.txt
diff /backup/running-services.txt /tmp/new-services.txt

# Check critical services
for service in sshd NetworkManager firewalld; do
    echo "Checking $service..."
    systemctl status $service --no-pager | head -3
done

# Test network connectivity
ping -c 3 google.com

Application Testing

# Test your applications
echo "🧪 Testing applications..."

# If you have a web server
if systemctl is-active --quiet httpd; then
    curl -I localhost
fi

# If you have a database
if systemctl is-active --quiet mysqld; then
    mysql -e "SELECT VERSION();"
fi

# Check listening ports
ss -tulpn | grep LISTEN

echo "✅ All applications working!"

🎮 Quick Examples

Example 1: Migrating a Web Server 🌐

# Complete web server migration example
echo "🌐 Migrating CentOS web server to AlmaLinux..."

# 1. Backup website data
tar -czf /backup/website-$(date +%Y%m%d).tar.gz /var/www/

# 2. Run migration
sudo ./almalinux-deploy.sh

# 3. Verify web server after migration
systemctl restart httpd
curl -I http://localhost

# 4. Test PHP if installed
php -v
echo "<?php phpinfo(); ?>" > /var/www/html/test.php
curl http://localhost/test.php | grep -i alma

echo "✅ Web server successfully migrated!"

Example 2: Migrating a Database Server 💾

# Database server migration with zero downtime
echo "💾 Migrating database server..."

# 1. Create database backup
mysqldump --all-databases --single-transaction > /backup/db-pre-migration.sql

# 2. Note current version
mysql -V

# 3. Perform migration
sudo ./almalinux-deploy.sh

# 4. Verify database after migration
systemctl status mysqld
mysql -e "SHOW DATABASES;"
mysql -e "SELECT @@version;"

# 5. Run test queries
mysql -e "SHOW STATUS LIKE 'Uptime';"

echo "✅ Database server migration complete!"

Example 3: Migrating a Docker Host 🐳

# Migrate Docker host from CentOS to AlmaLinux
echo "🐳 Migrating Docker host..."

# 1. List running containers
docker ps > /backup/docker-containers.txt

# 2. Save docker images
docker save $(docker images -q) -o /backup/docker-images.tar

# 3. Run migration
sudo ./almalinux-deploy.sh

# 4. Reinstall Docker
sudo dnf install -y docker-ce docker-ce-cli containerd.io

# 5. Restore and verify
docker load -i /backup/docker-images.tar
docker ps

echo "✅ Docker host successfully migrated!"

🚨 Fix Common Problems

Problem 1: Repository Errors During Migration ❌

Symptoms:

• “Cannot find valid baseurl for repo” errors
• Repository metadata download failures

Try this:

# Fix repository issues
sudo dnf clean all
sudo rm -rf /var/cache/dnf/*

# Manually update repo files
sudo curl -o /etc/yum.repos.d/almalinux.repo https://repo.almalinux.org/almalinux/8/almalinux.repo

# Retry the sync
sudo dnf distro-sync -y --allowerasing

Problem 2: Package Conflicts ❌

Try this:

# Resolve package conflicts
# Remove conflicting packages
sudo dnf remove -y centos-backgrounds centos-logos

# Force AlmaLinux packages
sudo dnf install -y almalinux-backgrounds almalinux-logos --allowerasing

# Clean and retry
sudo dnf clean all
sudo dnf distro-sync -y --skip-broken

Problem 3: Boot Issues After Migration ❌

Check these things:

# Regenerate initramfs
sudo dracut -f --regenerate-all

# Update GRUB configuration
sudo grub2-mkconfig -o /boot/grub2/grub.cfg

# Set default kernel
sudo grubby --set-default /boot/vmlinuz-$(uname -r)

# Verify boot entries
sudo grubby --info=ALL | grep -E "^kernel|^title"

📋 Simple Commands Summary

💡 Tips for Success

1. Always backup first 🌟 - Never skip the backup step, even for test systems
2. Test in staging 🔐 - Try migration on a test server before production
3. Schedule downtime 🚀 - Plan 1-2 hours of maintenance window
4. Monitor closely 📝 - Watch logs during migration for any issues
5. Keep documentation 🔄 - Document your specific migration steps and any customizations

🏆 What You Learned

Congratulations! Now you can:

• ✅ Understand why AlmaLinux is the best CentOS replacement
• ✅ Prepare systems properly for migration
• ✅ Perform seamless migration from CentOS to AlmaLinux
• ✅ Verify successful migration and troubleshoot issues
• ✅ Migrate various server types (web, database, Docker)

🎯 Why This Matters

Your migration to AlmaLinux provides:

• 🚀 Long-term stability with 10 years of guaranteed support
• 🔐 Enterprise-grade security with timely patches and updates
• 📊 Cost savings with no licensing or subscription fees
• ⚡ Community support from a vibrant, growing ecosystem

Remember: AlmaLinux is more than just a CentOS replacement - it’s a community-driven enterprise Linux that ensures your infrastructure remains stable, secure, and free forever! ⭐

You’ve successfully mastered CentOS to AlmaLinux migration! Your servers are now running on a rock-solid, community-backed enterprise Linux distribution that will serve you reliably for years to come! 🙌