With CentOS 6.0 now available on mirrors, many sysadmins face the dilemma of choosing between fresh installs and in-place upgrades. Having worked extensively with both RHEL 5.6 and 6.x in test environments, I've gathered some insights worth sharing.

Consider in-place upgrades when:

• Maintaining existing configurations is critical
• Downtime windows are extremely limited
• Complex application dependencies exist

The major upgrade hurdles include:

Package Compatibility Issues

# Check for obsolete packages
rpm -qa --qf '%{NAME}\n' | sort > installed-packages.txt
# Compare with CentOS 6 package list
comm -23 installed-packages.txt centos6-packages.txt

Configuration File Migration

Use rpmconf to handle config file changes:

yum install rpmconf
rpmconf -a

Service Compatibility

Test critical services before upgrade:

for service in httpd mysqld postfix; do
  service $service condrestart
done

1. Create complete system backup
2. Update all existing packages
3. Install preupgrade package:

yum install preupgrade
preupgrade-cli "CentOS 6"

Essential checks after upgrade:

# Verify kernel version
uname -r
# Check SELinux status
sestatus
# Validate repository configuration
yum repolist

For critical systems, consider:

# Set up new CentOS 6 server
virt-install --name centos6-mig \
--ram 2048 --disk path=/var/lib/libvirt/images/centos6-mig.qcow2 \
--location http://mirror.centos.org/centos/6/os/x86_64/

This approach allows gradual service migration while maintaining the old system as fallback.

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With CentOS 6.0 now available on mirrors, many sysadmins face the critical decision between fresh installation and in-place upgrade. Having worked extensively with both RHEL 5.6 and 6.x in production environments, I've compiled practical insights for this migration.

Consider in-place upgrades when:

• Maintaining existing configurations is critical
• Downtime windows are extremely limited
• Complex application dependencies exist

# Verify current OS version
cat /etc/redhat-release

# Check disk space (minimum 10GB free)
df -h

# Backup critical data
tar -czvf /backup/centos5_backup.tar.gz /etc /home /var/www

# List installed packages
rpm -qa > installed_packages.txt

Official documentation recommends using preupgrade tool:

# Install preupgrade utility
yum install preupgrade

# Run preupgrade check
preupgrade

# Download CentOS 6 packages
preupgrade-cli "CentOS 6.0"

Package Conflicts: Many CentOS 5 packages won't work on CentOS 6. Solution:

# Clean problematic packages
package-cleanup --problems
package-cleanup --orphans

Service Compatibility: Init scripts may need updating. Check critical services:

# Verify service status
service --status-all | grep running

# Example Apache config migration
cp /etc/httpd/conf/httpd.conf /etc/httpd/conf/httpd.conf.rhel5backup

# Verify OS version
cat /etc/redhat-release

# Check for failed services
systemctl --failed

# Test key functionality
curl -I localhost
mysql -V

For systems with:

• Custom kernel modules
• Proprietary hardware drivers
• Complex SELinux policies

Always document your upgrade process and have rollback plans ready. Test in staging environments before production upgrades.