Upgrading a 75TB SQL Server 2016 VLDB databases to SQL Server 2025 while using Always On Availability Groups (AGs) requires a structured approach to ensure minimal downtime, data integrity, and high availability. The process involves pre-upgrade assessments, testing, and careful execution in a production environment.
🔷 Prerequisites & Considerations
Understand SQL Server 2025 Compatibility Changes
Review SQL Server 2025 Release Notes & Deprecated Features.
Run the Microsoft Data Migration Assistant (DMA) to identify any breaking changes.
Check Always On AG feature support and enhancements in SQL Server 2025.
Perform a Full Backup & Disaster Recovery Planning
Full database, log, and system database backups (master, msdb, model).
Back up Always On AG configuration.
Ensure proper documentation of cluster configuration and quorum settings.
Check Storage, Performance & System Requirements
Ensure SQL Server 2025 is supported on the Windows Server version in use.
Verify sufficient disk space for temporary files and rollback operations.
Check that drivers, firmware, and OS updates are current.
Ensure High Availability Readiness
If using a geo-distributed AG, ensure replication is healthy.
Confirm network latency between primary and secondary replicas.
Test automatic failover scenarios.
🔷 Upgrade Approaches
There are two main approaches to upgrade a 25TB SQL Server 2016 database in an Always On Availability Group:
Rolling Upgrade (Preferred for Minimal Downtime)
Upgrade one secondary replica at a time while the primary remains active.
Perform failover to the upgraded secondary and then upgrade the old primary.
Best for minimizing downtime in production.
Side-by-Side Migration (If a Major Change is Needed)
Set up a new SQL Server 2025 cluster.
Use log shipping or database mirroring for synchronization.
Cutover to the new environment after verification.
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