Seamless Migration: How to Upgrade from CentOS 7 to AlmaLinux 8 with Zero Downtime

Many sysadmins and enterprise IT teams face a daunting challenge as CentOS 7 approaches its end-of-life: the critical need to migrate to a supported, stable, and secure operating system without disrupting business processes. The community’s trust is shifting towards AlmaLinux 8 — a robust RHEL-compatible alternative designed for production environments.

A common misconception is that moving from CentOS 7 to AlmaLinux 8 requires lengthy downtimes or complex multi-step overhauls. But what if you could upgrade smoothly with zero downtime? This guide will walk you through practical steps, tips, and examples ensuring your migration is both seamless and safe.

Why Migrate from CentOS 7 to AlmaLinux 8?

Before diving into the how, here’s the why:

End of Life Imminent: CentOS 7 will no longer receive updates or security patches after its official EOL date.
Security & Stability: AlmaLinux 8 continues to offer enterprise-grade support compatible with RHEL.
Community-Driven: Developers and enterprises trust its open governance and longevity commitment.
Minimal Learning Curve: Retain your existing workloads, configurations, and tooling with AlmaLinux’s binary compatibility.

Pre-Migration Checklist

Backup Everything
Ensure full system backups are taken, including configuration files, databases, custom scripts, and key directories (/etc, /var/lib, /home).
Inventory & Compatibility Check
List installed RPMs and third-party repos:

rpm -qa --qf '%{NAME}-%{VERSION}-%{RELEASE}.%{ARCH}\n' > installed_packages.txt

Verify compatibility of critical applications on AlmaLinux 8.

Prepare a Test Environment
Clone your setup in a VM or container for test runs before live migration.
Notify Stakeholders
Communicate the plan with relevant teams for monitoring during migration.

Zero Downtime Migration Strategy: Step-by-Step

The essence of zero downtime lies in careful sequencing — running both systems in parallel briefly, migrating workloads gracefully, then switching roles seamlessly.

Step 1: Set Up AlmaLinux 8 on Parallel Hardware or VM

Instead of upgrading in-place (which can be risky), deploy AlmaLinux 8 on another machine or virtual instance mimicking your production environment.

Step 2: Synchronize Data Continuously

Use tools like rsync or real-time syncing solutions (lsyncd) to mirror data directories from CentOS 7 to AlmaLinux in near real-time:

rsync -avz --delete /var/www/html/ almalinux-server:/var/www/html/

For databases like MySQL or PostgreSQL:

Set up replication between CentOS's database server and AlmaLinux's server.
Keep both DBs in sync during transition.

Step 3: Match Configurations and Services

On the AlmaLinux host:

Install all necessary packages matching CentOS:

dnf install @base-x @core @development-tools

Migrate config files from backups; validate them.
Adjust service configs if needed (paths can differ slightly).

Step 4: Test Applications on AlmaLinux

Before cutting over:

Point testers or staging traffic (DNS-based partial routing) towards the AlmaLinux server.
Verify app functionality, performance, and logs for errors.

Step 5: Switch Traffic Using Load Balancers or DNS

Once confident:

Update load balancer backend pools by adding AlmaLinux server behind LB.
Gradually shift traffic away from CentOS server over minutes/hours while monitoring metrics.

If no load balancer:

Change DNS entries TTL in advance to low values (~60 seconds).
Switch DNS IPs from CentOS IP to AlmaLinux IP during low traffic periods.

Step 6: Decommission Old Server Post-Migration

After all traffic stabilizes on AlmaLinux:

Gracefully shutdown services on CentOS.
Archive old system data for rollback if necessary.
Monitor new environment closely for at least one maintenance window.

Example: Migrating an Apache Web Server with MySQL Backend

Assume you run a standard LAMP stack on CentOS7:

On AlmaLinux VM:

dnf install httpd mysql-server php php-mysqlnd
systemctl enable --now httpd mysqld

Configure MySQL replication:

On CentOS (master):

CHANGE MASTER TO MASTER_LOG_FILE='mysql-bin.000001', MASTER_LOG_POS=107;

On AlmaLinux (slave):
```
START SLAVE;
```

Use rsync nightly sync for /var/www/html:

rsync -a /var/www/html/ almalinux:/var/www/html/

Point partial traffic using NGINX reverse proxy pointing half traffic at each backend; trend monitoring tools like New Relic or Nagios help ensure stability.

Final Thoughts

Migrating from CentOS 7 to AlmaLinux 8 can be accomplished without system downtime — it just requires thorough preparation, parallel environment setup, data replication, and gradual cutovers.

This approach minimizes business disruption while ensuring security patching continuity and future-proofing your infrastructure.

Your next mission? Start setting up that parallel AlmaLinux environment today!

If you found this guide helpful or have questions about specific workloads, drop a comment below — happy migrating!

Centos 7 To Almalinux 8