Microsoft Always On is a high-availability and disaster recovery feature introduced in SQL Server 2012. It is designed to provide both high availability (HA) and disaster recovery (DR) solutions, ensuring that SQL Server databases remain operational even in the event of hardware failure, network issues, or other types of failures. Always On offers two primary components: Always On Availability Groups and Always On Failover Cluster Instances (FCIs).
Here’s a detailed look at Microsoft Always On and its key components:
1. Always On Availability Groups (AGs)
Overview
Always On Availability Groups (AGs) is a high-availability feature that enables a set of databases to fail over together as a group. An Availability Group consists of a primary replica (the active instance) and one or more secondary replicas (standby instances). The primary replica handles all database reads and writes, while the secondary replicas provide real-time database copies that can be used for failover, reporting, and backup purposes.
Key Features of Always On Availability Groups:
- Multiple Databases: Unlike older SQL Server clustering methods, which were limited to a single database, Availability Groups support multiple databases in a single group.
- Automatic Failover: If the primary replica fails, one of the secondary replicas can be promoted to become the new primary automatically, minimizing downtime.
- Read-Only Secondaries: Secondary replicas can be configured as read-only, which allows for offloading read-only workloads (like reporting and backups) to secondary replicas.
- Synchronous and Asynchronous Replication:
- Synchronous Commit: Data is replicated to secondary replicas in real-time, ensuring that the data on the primary and secondary replicas is always consistent.
- Asynchronous Commit: Allows for replication to secondary replicas with some lag, providing more flexibility in terms of performance but with the potential risk of data loss in the event of a failover.
- Automatic and Manual Failover: You can configure automatic failover in synchronous replica setups or perform a manual failover when needed.
- Readable Secondaries for Reporting: Secondary replicas can be used for read-only queries, reducing the load on the primary server for reporting or read-heavy operations.
When to Use Always On Availability Groups:
- When you need high availability and disaster recovery for multiple databases.
- For applications requiring real-time database replication and automatic failover.
- When you want to offload reporting and backups to secondary replicas.
- For environments where minimal downtime and no data loss are essential.
Requirements for Always On Availability Groups:
- SQL Server Enterprise Edition (for most features).
- A Windows Server Failover Cluster (WSFC) for the underlying failover infrastructure.
- At least two SQL Server instances running in the same Active Directory domain.
- Database mirroring endpoints to communicate between replicas.
2. Always On Failover Cluster Instances (FCIs)
Overview
A Failover Cluster Instance (FCI) is another high-availability solution offered by Always On, and it provides failover at the entire SQL Server instance level, not just individual databases. FCIs rely on Windows Server Failover Clustering (WSFC) to enable a group of servers to act as a single unit. In case of a failure on one node, SQL Server will automatically failover to another node without significant downtime.
Key Features of Always On Failover Cluster Instances:
- Instance-Level Failover: Unlike Availability Groups, which operate at the database level, FCIs provide failover for the entire SQL Server instance, meaning that all databases within the instance fail over together.
- Shared Storage: FCIs rely on shared storage, typically using a SAN (Storage Area Network) or a clustered disk array. The storage is accessible by all nodes in the cluster, ensuring that the databases are available across all nodes.
- Automatic Failover: In case of hardware failure or a node going down, SQL Server automatically fails over to another node in the cluster without requiring manual intervention.
- Windows Server Failover Clustering: FCIs use the underlying Windows Server Failover Clustering technology to provide high availability, which ensures that there is minimal downtime during failovers.
When to Use Always On Failover Cluster Instances:
- When you need high availability for the entire SQL Server instance (not just specific databases).
- When shared storage is available, and you want to ensure that all databases in the instance fail over together.
- For scenarios where you need an always-on SQL Server instance with a shared storage configuration.
Requirements for Always On Failover Cluster Instances:
- SQL Server Enterprise Edition (for the full set of high-availability features).
- A Windows Server Failover Cluster (WSFC) to manage failover.
- Shared storage (typically from a SAN or clustered disk array) that all cluster nodes can access.
- A minimum of two nodes in the cluster.
- A dedicated cluster network for communication between the nodes.
3. Key Benefits of Always On
3.1 High Availability
- Always On ensures that SQL Server databases remain highly available, with automatic failover to minimize downtime in case of hardware or software failure. This is critical for business-critical applications where minimal downtime is essential.
3.2 Disaster Recovery
- Always On provides built-in disaster recovery capabilities by replicating data across multiple nodes or even across geographically separated data centers. In the event of a disaster, the secondary replicas can be promoted to become the new primary instance, ensuring business continuity.
3.3 Offload Read Queries and Backups
- Secondary replicas can be configured to handle read-only queries or backup operations, which helps offload workload from the primary replica and improve performance.
3.4 Flexible Replication Options
- Always On supports both synchronous and asynchronous replication, giving you the flexibility to balance between performance and data protection. Synchronous replication ensures that data is immediately available on secondary replicas, while asynchronous replication provides better performance but may introduce some latency.
3.5 Integration with Azure
- With the advent of SQL Server on Azure, Always On Availability Groups can be integrated with Azure’s infrastructure to create highly available solutions across on-premises and cloud environments.
4. Limitations and Considerations
4.1 Licensing
- Always On Availability Groups require SQL Server Enterprise Edition, which can be costly compared to the Standard Edition. Similarly, Failover Cluster Instances also require the Enterprise Edition for certain advanced features.
4.2 Complexity
- Setting up Always On can be complex, particularly when configuring the underlying Windows Server Failover Cluster and managing the replicas. Ensuring that your environment meets all prerequisites and that you have the appropriate network and storage configurations is essential.
4.3 Shared Storage for FCIs
- Always On FCIs require shared storage, which can introduce complexity in terms of hardware setup and management. If you’re using FCIs, you need to ensure that your storage infrastructure is properly designed for high availability.
4.4 Network Latency and Bandwidth
- In a geographically distributed Always On Availability Group setup, network latency and bandwidth between the primary and secondary replicas can impact performance. It’s crucial to assess the network infrastructure and ensure that it can handle the replication load.
4.5 No Cross-Version Support
- Always On Availability Groups do not support cross-version replication between different versions of SQL Server. Both the primary and secondary replicas must run the same version of SQL Server.
5. Best Practices for Implementing Always On
5.1 Proper Sizing and Configuration
- Properly size your SQL Server instances, replicas, and storage based on the workload. Choose the appropriate instance types, storage types (e.g., SSDs for high I/O workloads), and network configurations to meet your performance and availability needs.
5.2 Backup Strategy
- Even though Always On provides high availability and disaster recovery, you should still have a comprehensive backup strategy that includes regular backups of all your databases, transaction logs, and system databases.
5.3 Monitoring and Alerting
- Regularly monitor the health of your Always On Availability Groups or Failover Cluster Instances. Use SQL Server Management Studio (SSMS), SQL Server Profiler, or third-party monitoring tools to track performance, replication status, and failover events.
5.4 Testing Failover Scenarios
- Regularly test the failover process to ensure that it works as expected in the event of a disaster or failure. This includes testing both manual and automatic failovers.
5.5 Network and Storage Considerations
- Ensure that your network between replicas and nodes is low-latency and high-bandwidth. Additionally, for FCIs, ensure that your shared storage system is configured for redundancy and high availability.
Conclusion
Microsoft Always On is a robust and flexible solution for ensuring high availability and disaster recovery for SQL Server databases. Depending on your needs, you can choose between Always On Availability Groups for database-level failover and Always On Failover Cluster Instances for instance-level failover. With its automatic failover, read-write splitting, and integration with Azure, Always On provides a powerful platform for mission-critical SQL Server applications, ensuring that your database systems remain resilient in the face of failures and disasters. However, it’s important to consider the complexity, licensing costs, and infrastructure requirements when implementing Always On.