Implementing High Availability with SQL Server Always On
High availability is a pivotal concern for organizations of all sizes. In today’s data-driven world, databases form the backbone of most business applications. A brief downtime can result in significant financial loss and damage to reputation. Microsoft SQL Server provides a robust solution for high availability through its Always On feature, which ensures continuous data availability and minimal downtime. This blog entry aims to provide a comprehensive analysis of how to implement high availability using SQL Server Always On technology.
Understanding High Availability Concepts
Before diving into the specifics of SQL Server Always On, it is important to understand the concept of high availability. In essence, high availability refers to the measures used to ensure that a system is operational and accessible for a predetermined amount of time. This is usually expressed in terms of a percentage called the uptime. For example, a system with a high availability of 99.999% — known as ‘five nines’ — means the system is designed to be down for no more than 5.26 minutes per year.
High availability can be achieved through various means, including failover clustering, database mirroring, and log shipping. Of these methods, SQL Server Always On offers comprehensive benefits, which we’ll examine in more detail.
What is SQL Server Always On?
SQL Server Always On is a high availability and disaster recovery solution introduced in Microsoft SQL Server 2012. It enhances existing SQL Server features with additional capabilities such as failover cluster instances and availability groups. By leveraging Always On, businesses can achieve high levels of data availability and meet various recovery objectives.
Components of SQL Server Always On
SQL Server Always On is comprised of two key components, which are:
- Always On Failover Cluster Instances (FCIs): This component utilizes Windows Server Failover Clustering (WSFC) to maintain the availability of SQL Server instances. In the event of a hardware or software failure, the operations are quickly moved to another node within the failover cluster, thus ensuring minimal downtime.
- Always On Availability Groups (AGs): AGs allow a set of user databases, termed as availability databases, to fail over together. Availability groups support a primary database which is read-write and up to eight secondary databases that can be used for read-only access and backup operations.
Together, these components provide a robust approach to implementing high availability in SQL Server environments.
Benefits of Using SQL Server Always On
There are multiple benefits to using SQL Server Always On, including:
- Enhanced Availability: Always On maximizes the uptime of your databases, thus ensuring that your critical applications remain available during unplanned and planned outages.
- Improved Disaster Recovery: With the capability to have secondary replicas in geographically dispersed locations, Always On ensures that your data can survive major disasters.
- Decreased downtime during maintenance: Always On allows you to perform certain maintenance tasks on secondary replicas without affecting the primary database.
- Read-Only Access to Secondary Replicas: This can improve performance for reporting and backup operations by offloading these tasks to secondary replicas.
- Automatic Page Repair: Always On automatically tries to repair corrupted pages by requesting a fresh copy from another replica.
The combined benefits essentially mean that SQL Server Always On enables organizations to maintain higher service levels and meet their business requirements effectively.
Prerequisites for Implementing Always On
Before deploying Always On, there are several prerequisites that must be addressed:
- Windows Server Failover Clustering (WSFC): A WSFC with all required SQL servers as nodes must be set up. Properly configured WSFC is critical to the success of Always On implementations.
- Windows Server Versions: It’s important to ensure that the version of Windows Server being used is compatible with the SQL Server version for Always On.
- SQL Server Edition and Version: Always On is available only on SQL Server Enterprise Edition, starting from SQL Server 2012 onwards.
- Network Configuration: A stable and secure network is necessary for nodes to communicate within the WSFC setup.
- Domain Accounts: Service accounts should have the necessary permissions for SQL Server and WSFC.
- Synchronous Commit Mode: The data on secondary replicas may be up to date with the primary replica when using synchronous commit mode, this is essential for automatic failover.
- SQL Server Endpoints: Communication between Availability Group replicas is handled over SQL Server endpoints, usually using TCP. Proper configuration of these endpoints is mandatory.
All these components need proper setup and configuration to ensure a smooth and successful deployment of SQL Server Always On.
Steps to Implement SQL Server Always On
Step 1: Prepare Windows Server Failover Clustering
Begin by configuring the WSFC environment. This involves setting up the failover clusters, creating a Cluster Name Object (CNO), and verifying network configurations and quorum settings. The WSFC cluster should be validated to make sure that it meets all requirements for SQL Server Always On AGs.
Step 2: Install SQL Server Instances
The SQL Server instances that will participate in the Always On Availability Group must be installed on the nodes of the WSFC. During installation, it is crucial to select the ‘SQL Server Always On feature’, enabling support for availability groups in the SQL Server instance.
Step 3: Configure SQL Server Always On Availability Groups
Create a new availability group by using SQL Server Management Studio or PowerShell scripts. For the new group, specify the databases to include, the replicas, the desired failover modes, and read access settings on secondary replicas.
Step 4: Verify Configuration and Failover Readiness
Once your Availability Group is configured, you can verify its health and readiness for failover by checking the dashboard in SQL Server Management Studio, which provides a visual representation of the current state of each replica and database in the Availability Group.
Step 5: Perform Failover Testing
Testing is a crucial step in any high availability implementation. It’s important to execute planned manual failovers among the replicas to ensure that everything is working correctly. In addition to manual failovers, further testing for automatic failover, data synchronization, and other key scenarios should be carried out.
Maintenance and Monitoring of Always On AGs
Post-deployment, it’s important to have monitoring in place to ensure system health and proactively address issues. SQL Server provides various tools for monitoring the health of your Always On Availability Groups, such as:
- Always On Dashboard
- SQL Server Management Studio (SSMS) Reports
- System-center-based monitoring
- Custom scripts to check failover readiness and alerts
Regular maintenance tasks…
