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Upgrading an operating system is always a critical step that requires careful preparation and understanding of risks involved. Transitioning from CentOS 7 to CentOS 8 is no exception and requires a thorough plan. It's important to note that starting from CentOS 8, the project has shifted towards CentOS Stream, which is a rolling-release distribution serving as a precursor to RHEL (Red Hat Enterprise Linux). While the process may seem daunting, adhering to the steps outlined below will ensure a successful upgrade.

Preparation for Upgrade

1. Data Backup: The most crucial step that should never be overlooked. Backup all important files, databases, and configurations.

2. Update Existing System: Before initiating the upgrade, it's important to have CentOS 7 fully updated. Run yum update and reboot the system.

3. Check Compatibility: Ensure that your hardware and software are compatible with CentOS 8. This may involve checking with hardware manufacturers and software developers.

4. Documentation of Changes: CentOS 8 brings many changes, including newer versions of software and changes in system management. Familiarize yourself with these changes to avoid surprises.

Upgrade Process

Upgrading from CentOS 7 to CentOS 8 requires several specific steps, as direct upgrade via yum is not officially supported. Here's a general procedure to follow:

1. Install DNF Tool: CentOS 8 uses DNF as the package manager instead of YUM. Though YUM is still available, DNF is the default and offers better performance and features.

   • Remove YUM:

     yum remove yum yum-plugin-fastestmirror
     ​

   • Install DNF:

     yum install dnf
     ​

2. Remove Unsupported Packages: Some CentOS 7 packages may not be compatible with CentOS 8. Use dnf to identify and remove them.

3. Add CentOS 8 Repositories: Remove CentOS 7 repositories and add CentOS 8 repositories. This ensures that you'll be installing packages from the new system during further steps.

4. Upgrade the System: Now you can proceed with upgrading the system using DNF. This step may take some time depending on the number of updates and internet connection speed.

   dnf upgrade
   

5. Resolve Dependencies: During the upgrade, there may be dependency issues with packages. DNF offers tools to resolve them, but manual intervention may be required in some cases.

6. Final Check and Reboot: After the upgrade completes, carefully check the system and logs for any unresolved errors or warnings. Finally, reboot the system to apply the changes.

Post-Upgrade

1. Functionality Testing: Test important services and applications to ensure everything is functioning as expected.

2. Configuration of New Services: CentOS 8 may include new services or changes in the configuration of existing services. Make necessary configuration adjustments.

3. System Monitoring: Monitor system logs and performance to identify any potential issues arising post-upgrade.

Upgrading from CentOS 7 to CentOS 8 is a complex process that requires thorough preparation and caution. By following these steps, you'll ensure your system is successfully updated with minimal interruptions.