Always On Availability Groups vs. Database Mirroring: Which One to Use?

Introduction

When it comes to high availability and disaster recovery solutions for SQL Server, two technologies often come into consideration: Always On Availability Groups (AGs) and Database Mirroring. While both serve similar purposes, they have significant differences in capabilities, complexity, and use cases. This article will compare these two technologies to help you determine which solution best fits your organization's needs.

Technology Overview

Database Mirroring

Database Mirroring was introduced in SQL Server 2005 as a solution for high availability and disaster recovery. It operates at the database level, maintaining two copies of a single database (principal and mirror) that reside on different server instances.

Always On Availability Groups

Always On Availability Groups (AGs) were introduced in SQL Server 2012 as a more robust alternative. AGs provide enterprise-level high availability and disaster recovery solutions, working with groups of databases (rather than individual databases) and supporting multiple secondaries.

Feature Comparison

Feature	Database Mirroring	Always On Availability Groups
Scope	Single database	Group of databases
Secondary Usage	Read-only (high safety only)	Readable secondaries (all modes)
Failover Type	Automatic (with witness) or manual	Automatic or manual
Number of Replicas	1 principal, 1 mirror (+1 witness)	Up to 8 secondaries (9 total)
Backup on Secondary	No	Yes
Monitoring	Basic	Integrated dashboard
Encryption Support	Limited	Full TDE support
SQL Server Edition	Standard/Enterprise	Enterprise only
Listener Support	No	Yes
Automatic Page Repair	Yes	Yes
Compression	No	Yes

Performance Considerations

Database Mirroring

• Network Impact: Synchronous mirroring can introduce latency as transactions must commit on both servers

• Resource Usage: Generally lighter weight than AGs but less efficient with bandwidth

• Throughput: Limited by single-threaded log stream

Always On Availability Groups

• Parallel Processing: Multiple databases can failover together

• Network Efficiency: Better compression and more efficient log block transfer

• Read Scaling: Offloads read workloads to secondary replicas

• Resource Requirements: More memory and CPU intensive due to additional functionality

Deployment Scenarios

When to Choose Database Mirroring

1. Legacy Systems: Supporting older versions of SQL Server (pre-2012)

2. Simple HA Needs: Single database protection with basic requirements

3. Standard Edition: When Enterprise Edition isn't available

4. Limited Resources: When server resources are constrained

5. Quick Setup: For temporary or proof-of-concept solutions

When to Choose Always On Availability Groups

1. Enterprise Environments: Mission-critical systems requiring maximum uptime

2. Multiple Databases: Protecting groups of related databases together

3. Read Scaling: Offloading reporting workloads to secondaries

4. Advanced DR: Multi-site disaster recovery configurations

5. Modern Features: Need for features like readable secondaries, automatic page repair, etc.

Migration Considerations

From Mirroring to Always On

1. Version Requirements: AGs require SQL Server 2012 or later, Enterprise Edition

2. Topology Changes: AGs use Windows Server Failover Clustering (WSFC)

3. Client Connectivity: AG listeners provide more flexible connection options

4. Monitoring: AGs integrate with SQL Server Management Studio dashboards

5. Feature Expansion: AGs support distributed availability groups for extended DR

Best Practices

For Database Mirroring

• Use high-safety mode with automatic failover for critical systems

• Monitor the mirroring status and performance counters regularly

• Consider using a witness server for automatic failover capability

• Test failovers regularly to ensure proper operation

For Always On Availability Groups

• Configure appropriate availability modes (synchronous vs. asynchronous) based on distance

• Distribute replicas across different physical locations for disaster protection

• Leverage readable secondaries to offload reporting workloads

• Implement proper quorum configurations for WSFC

• Monitor synchronization health and performance metrics

Limitations to Consider

Database Mirroring Limitations

• Only one mirror server per principal database

• Mirror database is inaccessible in high-performance mode

• Limited monitoring capabilities

• No support for FILESTREAM data in mirrored databases

• No cross-platform support (Windows only)

Always On Limitations

• Enterprise Edition requirement for most features

• More complex setup and configuration

• WSFC dependency adds management overhead

• Storage must be independent for each replica

• Certain features (like CDC) have limitations on secondaries

Future Outlook

Microsoft has announced that Database Mirroring will be deprecated in a future version of SQL Server. While it remains supported in current versions, organizations should consider migrating to Always On Availability Groups for long-term solutions. Always On continues to receive new features and improvements with each SQL Server release.

Decision Flowchart

1. Is this for SQL Server 2012+ Enterprise Edition?

   • Yes → Consider Always On

   • No → Database Mirroring may be your only option

2. Do you need to protect multiple databases together?

   • Yes → Always On

   • No → Proceed to next question

3. Do you need readable secondaries?

   • Yes → Always On

   • No → Proceed to next question

4. Is this a temporary or non-critical solution?

   • Yes → Database Mirroring may suffice

   • No → Always On

Conclusion

While Database Mirroring served as a reliable high availability solution for many years, Always On Availability Groups represent the modern, more capable alternative for most enterprise scenarios. The choice between them depends on your SQL Server version, edition, specific requirements, and long-term plans.

For new deployments on supported versions, Always On Availability Groups are generally the preferred choice due to their superior functionality, scalability, and Microsoft's ongoing investment in the technology. Database Mirroring remains a viable solution for older systems or simpler requirements, but organizations should plan to migrate to Always On for future-proofing their high availability strategies.

Ultimately, the right solution depends on your specific business requirements, technical constraints, and operational capabilities. Evaluate both options carefully against your organization's needs to make the best choice for your environment.