Optimizing SQL Server for high availability

Every second counts when you’re running an e-commerce platform, a financial institution, or a logistics company, and it’s crucial to maintain service availability. With SQL Server, you can ensure that your organization’s mission-critical databases are always available. This robust, reliable, and widely used relational database management system underpins many enterprise-level applications driving today’s fast-paced business environment. However, it can require high availability to keep operations stable.

SQL Server offers several options to tune performance, each with unique strengths and weaknesses. However, making the right choice isn’t easy. You must consider your budget, resources, and performance goals before deciding between database mirroring, clustering, or Always On availability groups.

This article reviews the various options to help you understand which option is right for your use case, whether you’re a seasoned database administrator or just getting started with SQL. It explores the benefits of implementing high availability in SQL, including improved performance, reduced downtime, and better disaster recovery.

Achieving high availability in SQL Server

Achieving high SQL Server availability is essential for businesses that require uninterrupted access to their data to make informed decisions, meet customer demands, and stay ahead of their competition. It provides your stakeholders with 24/7 access to critical business data without downtime, data loss, or performance degradation. Let’s discuss some of the most popular methods.

Database mirroring

The database mirroring technique creates and maintains an exact copy of the principal (primary) server’s database on a secondary server, called a mirror server. The database mirroring process continuously sends transaction logs from the principal server to the mirror server, applying these changes to the database copy in real time.

If there’s a failure on the principal server, the mirror server automatically takes over as the primary server with minimal or no data loss. This automatic failover transpires within seconds. After the failover, the former mirror server becomes the new principal server and accepts client connections.

Database mirroring has been a popular choice for smaller environments due to its ease of setup and management. Administrators configure the feature using the SQL Server Management Studio and use it with other SQL Server high availability features for additional protection.

However, the overhead of mirroring transactions increases the server’s workload. Consequently, database mirroring provides limited scalability and can cause performance issues. Additionally, Microsoft has deprecated database mirroring since SQL Server 2012, making it unavailable in the latest versions. Microsoft recommends using Always On availability groups or basic availability groups instead.


Clustering unifies multiple servers into a single unit for improved availability and performance. This technique provides high availability and load balancing in SQL Server. While implementing clustering can be complex, the rewards are worth the effort in larger environments that can access shared storage.

SQL Server has two types of clustering: failover clustering and load balancing clustering.

Failover clustering

Failover clustering promotes high availability by using multiple servers to host a SQL Server instance. In this technique, these servers work together as a cluster to run a single SQL Server instance.

If one server in the cluster fails, another server takes over automatically to provide continued service. Consequently, the SQL Server instance is always available to clients.

Load balancing clustering

Load balancing clustering distributes the workload across multiple servers. This technique configures multiple servers to host multiple instances of SQL Server, and a load balancer distributes the workload among the instances. The load balancer monitors the workload and redirects incoming requests to the instance with the smallest workload. This improves performance and scalability.

Always On availability

SQL Server 2012 introduced Always On availability groups. This feature provides a high availability and disaster recovery solution for user databases called availability databases.

With Always On, users can create a group of up to eight databases that fail over together to a secondary replica in case of hardware, software, or network failures.

These Always On availability groups combine synchronous and asynchronous data replication to keep the primary and secondary replicas synchronized. The synchronous replication mode ensures that data is committed to the primary and secondary replicas before considering the transaction complete. This approach provides high data protection and minimal data loss.

However, the synchronous mode may introduce latency, impacting application performance.

The asynchronous mode provides higher performance but may result in data loss if a failure occurs before replicating the data to the secondary replica.

In addition to SQL Server high availability, Always On availability groups enable scaling out read workloads with readable secondary replicas. Readable secondary replicas provide read-only access to user databases for tasks such as reporting or analytics. Secondary replicas in geographically distant data centers allow users to access data in different regions, improving application performance and reducing data latency.

While Always On availability groups provide high availability and scalability, they require SQL Server Enterprise. Administrators need specialized knowledge to set up these complex groups. They must use the Windows Server failover clustering to manage the availability group and shared storage for quorum data. This is a limitation in some environments. However, the SQL Server Management Studio simplifies the process by providing a graphical user interface for setting up and configuring the availability group.

Best practices

Implementing high availability in SQL Server requires careful planning and execution. Best practices include:

  • Plan for high availability: First, assess your business requirements and identify the most critical databases that must be highly available. This approach helps determine which high availability method suits your organization and avoids unnecessary complexity.
  • Choose the right high availability method: SQL Server supports various high availability methods. Select the right one depending on your business needs, budget, and IT infrastructure.
  • Test your high availability solution: Testing your high availability solution regularly helps identify and resolve issues before they cause an outage. To inititate failover testing, use tools like SQLIOSim to simulate SQL Server Input/Output (I/O) activity or SQL Server Management Studio.
  • Monitor your high availability solution: Monitoring your high availability solution helps you detect issues and prevent downtime. You can track the health and performance of your SQL Server environment with SQL Server Management Studio, SQL Server Performance Dashboard, or more robust third-party monitoring tools.
  • Keep your high availability solution current: It's essential to update your SQL Server environment with the latest service packs, cumulative updates, and hotfixes. These updates help you avoid known issues and improve the stability of your high-availability solution. Furthermore, all servers in your configuration should run the same version and edition of SQL Server to avoid complications.
  • Maintain a disaster recovery plan: Despite high availability measures, disasters can still occur. A disaster recovery plan that includes regular backups and off-site storage helps you recover from a catastrophic event and minimize data loss.


Using high availability methods with SQL Server helps make your mission-critical production databases more reliable. Database mirroring — the first option — is easy to set up but has limited scalability. Additionally, Microsoft has deprecated this option. Clustering — the second option — provides high availability and load balancing, but the setup is difficult. It also requires shared storage.

The third option, Always On availability groups, provide high availability and disaster recovery for a group of databases. This method offers synchronous and asynchronous data replication and the ability to scale out read workloads with secondary replicas. However, these availability groups are complex to set up and require SQL Server Enterprise software.

High availability helps ensure that your data is always there when needed, improving your service’s reliability and giving you peace of mind. Review your specific requirements and budget to decide which option is best for you. Then, monitor your availability with Site24x7 and ensure your end-user experience meets your and your clients’ standards.

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