If a major change is required, such as hardware upgrades, OS changes, or cluster reconfigurations, a Side-by-Side Migration is the best approach. This method minimizes risk by keeping the old system intact while setting up a new Always On Availability Group (AG) on SQL Server 2025.
🔷 Overview of the Side-by-Side Migration Process
Prepare the New SQL Server 2025 Environment
Back Up & Restore Databases to the New Server
Set Up Always On AG on the New SQL Server 2025 Cluster
Synchronize Data Between Old and New Servers
Perform Application & Performance Testing
Cut Over to the New SQL Server 2025 Cluster
Decommission the Old SQL Server 2016 Cluster
🔷 Step-by-Step Upgrade Process
🔹 Step 1: Prepare the New SQL Server 2025 Environment
✅ Provision New SQL Server 2025 Cluster
✅ Configure Windows Failover Clustering (WSFC)
✅ Enable Always On Availability Groups on New Nodes
Enable Always On at the SQL Server level and Restart SQL Server.
✅ Ensure SQL Server Service Accounts Have Proper Permissions
Provide "Log on as a service" permission.
Ensure SQL Server and Cluster service accounts have access to: Database files and WSFC cluster resources
✅ Pre-Upgrade Compatibility Checks
Run Microsoft Data Migration Assistant (DMA) to analyze the SQL 2016 database for compatibility with SQL 2025.
🔹 Step 2: Backup & Restore Databases to the New Server
✅ Take a Full Backup on SQL Server 2016 Primary Node
✅ Restore the Backup to SQL Server 2025
✅ Keep the Database in NORECOVERY Mode
This allows future log shipping from the old system to keep the new database in sync until cutover.
🔹 Step 3: Set Up Always On Availability Group on SQL Server 2025
✅ Create a New Availability Group on SQL Server 2025
✅ Join Secondary Replicas to the AG
✅ Join Databases to AG
✅ Create an AG Listener
✅ Verify AG Health
🔹 Step 4: Synchronize Data Between Old & New Servers
✅ Set Up Log Shipping from SQL Server 2016 to SQL Server 2025
Take new log backups on the old system:
Restore logs to SQL Server 2025 (repeatedly until cutover):
Automate log shipping using SQL Agent Jobs.
✅ Monitor Synchronization
✅ Ensure No Outstanding Transactions
Run DBCC OPENTRAN on SQL Server 2016 to ensure no active transactions remain.
🔹 Step 5: Perform Application & Performance Testing
✅ Configure a Staging Environment
Point test applications to the new AG listener.
Run SQL Server Profiler to compare performance between old and new environments.
✅ Test Always On Automatic Failover
Manually fail over to a secondary replica to validate:
Ensure applications remain connected.
✅ Validate Database Integrity
🔹 Step 6: Cut Over to the New SQL Server 2025 Cluster
Step 7: Decommission the Old SQL Server 2016 Cluster
✅ Monitor the New SQL Server 2025 Cluster for 24-48 Hours
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