Prerequisites, restrictions, and recommendations for Always On availability groups

Applies to: SQL Server

This article describes considerations for deploying Always On availability groups, including prerequisites, restrictions, and recommendations for host computers, Windows Server failover clusters (WSFC), server instances, and availability groups. For each of these components security considerations and required permissions, if any, are indicated.

.NET hotfixes that support availability groups

Depending on the SQL Server components and features you'll use with Always On availability groups, you might need to install additional .NET hotfixes identified in the following table. The hotfixes can be installed in any order.

Checklist: Requirements (Windows system)

To support the Always On availability groups feature, ensure that every computer that is to participate in one or more availability groups meets the following fundamental requirements:

Recommendations for computers that host availability replicas (Windows system)

• Comparable systems: For a given availability group, all the availability replicas should run on comparable systems that can handle identical workloads.

• Dedicated network adapters: For best performance, use a dedicated network adapter (network interface card) for Always On availability groups.

• Sufficient disk space: Every computer on which a server instance hosts an availability replica must possess sufficient disk space for all the databases in the availability group. Keep in mind that as primary databases grow, their corresponding secondary databases grow the same amount.

• Identical disk layout: Every computer on which a server instance hosts an availability replica should have an identical disk layout (with exact disk drive letters and sizes) to ensure file paths for database files (mdf, ldf) are mirrored, preventing complications during seeding and synchronization. Review Restrictions (availability databases) for disk layouts that differ.

Permissions (Windows system)

To administer a WSFC, the user must be a system administrator on every cluster node.

For more information about the account for administering the cluster, see Appendix A: Failover Cluster Requirements.

Related tasks (Windows system)

Change the HostRecordTTL (using PowerShell)

1. Open PowerShell window via Run as Administrator.

2. Import the FailoverClusters module.

3. Use the Get-ClusterResource cmdlet to find the Network Name resource, then use Set-ClusterParameter cmdlet to set the HostRecordTTL value, as follows:

   Get-ClusterResource "<NetworkResourceName>" | Set-ClusterParameter HostRecordTTL <TimeInSeconds>

   The following PowerShell example sets the HostRecordTTL to 300 seconds for a Network Name resource named SQL Network Name (SQL35).

   Import-Module FailoverClusters
   
   $nameResource = "SQL Network Name (SQL35)"
   Get-ClusterResource $nameResource | Set-ClusterParameter HostRecordTTL 300
   

   Tip

   Every time you open a new PowerShell window, you need to import the FailoverClusters module.

Related content (PowerShell)

• Clustering and High-Availability (Failover Clustering and Network Load Balancing Team Blog)

• Getting Started with Windows PowerShell on a Failover Cluster

• Cluster resource commands and equivalent Windows PowerShell cmdlets

Related content (Windows system)

• Configure DNS settings in a Multi-Site Failover Cluster

• DNS Registration with Network Name Resource

SQL Server instance prerequisites and restrictions

Each availability group requires a set of failover partners, known as availability replicas, which are hosted by instances of SQL Server. A given server instance can be a stand-alone instance or a SQL Server failover cluster instance (FCI).

In this section:

• Checklist: Prerequisites
• Thread Usage by availability groups
• Permissions
• Related Tasks
• Related Content

Checklist: Prerequisites (server instance)

For a list of features supported by the editions of SQL Server on Windows, see:

• Editions and supported features of SQL Server 2022
• Editions and supported features of SQL Server 2019
• Editions and supported features of SQL Server 2017
• Editions and supported features of SQL Server 2016

Thread usage by availability groups

Always On availability groups has the following requirements for worker threads:

• On an idle instance of SQL Server, Always On availability groups uses 0 threads.

• The maximum number of threads used by availability groups is the configured setting for the maximum number of server threads ('max worker threads') minus 40.

• The availability replicas hosted on a given server instance share a single thread pool in SQL Server 2019 (15.x) and previous versions.

  Threads are shared on an on-demand basis, as follows:

  • Typically, there are 3-10 shared threads, but this number can increase depending on the primary replica workload.

  • If a given thread is idle for a while, it's released back into the general SQL Server thread pool. Normally, an inactive thread is released after ~15 seconds of inactivity. However, depending on the last activity, an idle thread might be retained longer.

  • A SQL Server instance uses up to 100 threads for parallel redo for secondary replicas. Each database uses up to one-half of the total number of CPU cores, but not more than 16 threads per database. If the total number of required threads for a single instance exceeds 100, SQL Server uses a single redo thread for every remaining database. Serial redo threads are released after approximately 15 seconds of inactivity.

• In addition, availability groups use unshared threads, as follows:

  • Each primary replica uses 1 Log Capture thread for each primary database. In addition, it uses 1 Log Send thread for each secondary database. Log send threads are released after ~15 seconds of inactivity.

  • A backup on a secondary replica holds a thread on the primary replica for the duration of the backup operation.

• SQL Server 2022 (16.x) introduced the parallel redo thread pool, which is an instance-level thread pool shared with all databases having redo work. With this pool, the same set of threads can process the log records for different databases at the same time (in parallel). In SQL Server 2019 (15.x) and previous versions, the number of available threads for redo is limited to 100.

• SQL Server 2019 (15.x) introduced parallel redo for memory optimized availability group databases. In SQL Server 2016 (13.x) and SQL Server 2017 (14.x), disk-based tables don't use parallel redo if a database in an availability group is also memory optimized.

For more information, see Always On - HADRON Learning Series: Worker Pool Usage for HADRON Enabled Databases (a CSS SQL Server Engineers Blog).

Permissions (server instance)

Related tasks (server instance)

Related content (server instance)

• Always On - HADRON Learning Series: Worker Pool Usage for HADRON Enabled Databases

Network connectivity recommendations

We strongly recommend that you use the same network links for communications between WSFC nodes and communications between availability replicas. Using separate network links can cause unexpected behaviors if some of links fail (even intermittently).

For example, for an availability group to support automatic failover, the secondary replica that is the automatic-failover partner must be in the SYNCHRONIZED state. If the network link to this secondary replica fails (even intermittently), the replica enters the UNSYNCHRONIZED state and can't begin to resynchronize until the link is restored. If the WSFC requests an automatic failover while the secondary replica is unsynchronized, automatic failover doesn't occur.

Client connectivity support

For information about Always On availability groups support for client connectivity, see Driver and client connectivity support for availability groups.

Prerequisites and restrictions for using a SQL Server failover cluster instance (FCI) to host an availability replica

In this section:

• Restrictions
• Checklist: Prerequisites
• Related Tasks
• Related Content

Restrictions (FCIs)

• The cluster nodes of an FCI can host only one replica for a given availability group: If you add an availability replica on an FCI, the WSFC nodes that are possible FCI owners can't host another replica for the same availability group. To avoid possible conflicts, it's recommended to configure possible owners for the failover cluster instance. This prevents potentially causing a single WSFC from attempting to host two availability replicas for the same availability group.

  Furthermore, every other replica must be hosted by an instance of SQL Server that resides on a different cluster node in the same Windows Server failover cluster. The only exception is that while being migrated to another cluster, an availability group can temporarily straddle two clusters.

  Warning

  Using the Failover Cluster Manager to move an FCI hosting an availability group to a node that is already hosting a replica of the same availability group might result in the loss of the availability group replica, preventing it from being brought online on the target node. A single node of a failover cluster can't host more than one replica for the same availability group. For more information on how this occurs, and how to recover, see the blog Replica unexpectedly dropped in availability group.

• FCIs don't support automatic failover by availability groups: FCIs don't support automatic failover by availability groups, so any availability replica that is hosted by an FCI can be configured for manual failover only.

• Changing FCI network name: If you need to change the network name of an FCI that hosts an availability replica, you'll need to remove the replica from its availability group and then add the replica back into the availability group. You can't remove the primary replica, so if you're renaming an FCI that is hosting the primary replica, you should fail over to a secondary replica and then remove the former primary replica and add it back. Renaming an FCI might alter the URL of its database mirroring endpoint. When you add the replica ensure that you specify the current endpoint URL.

Checklist: Prerequisites (FCIs)

Related tasks (FCIs)

Related content (FCIs)

• Failover Clustering and Always On Availability Groups (SQL Server)

• Always On Architecture Guide: Building a High Availability and Disaster Recovery Solution by Using Failover Cluster Instances and availability groups

Availability group prerequisites and restrictions

In this section:

• Restrictions
• Requirements
• Security
• Related Tasks

Restrictions (availability groups)

• Availability replicas must be hosted by different nodes of one WSFC: For a given availability group, availability replicas must be hosted by server instances running on different nodes of the same WSFC. The only exception is that while being migrated to another cluster, an availability group can temporarily straddle two clusters.

  Note

  Virtual machines on the same physical computer can each host an availability replica for the same availability group because each virtual machine acts as a separate computer.

• Unique availability group name: Each availability group name must be unique on the WSFC. The maximum length for an availability group name is 128 characters.

• Availability replicas: Each availability group supports one primary replica and up to eight secondary replicas. All of the replicas can run under asynchronous-commit mode, or up to five of them can run under synchronous-commit mode (one primary replica with two synchronous secondary replicas). Each replica must have a unique server name in both Windows and SQL Server. The server names between Windows and SQL Server must match.

• Maximum number of availability groups and availability databases per computer: The actual number of databases and availability groups you can put on a computer (VM or physical) depends on the hardware and workload, but there's no enforced limit. Microsoft has tested up to 10 AGs and 100 DBs per physical machine, however this isn't a binding limit. Depending on the hardware specification on the server and the workload, you can put a higher number of databases and availability groups on an instance of SQL Server. Signs of overloaded systems can include, but aren't limited to, worker thread exhaustion, slow response times for availability group system views and DMVs, and/or stalled dispatcher system dumps. Please make sure to thoroughly test your environment with a production-like workload to ensure it can handle peak workload capacity within your application SLAs. When considering SLAs be sure to consider load under failure conditions as well as expected response times.

• Don't use the Failover Cluster Manager to manipulate availability groups. The state of a SQL Server FCI is shared between SQL Server and the Windows Server Failover Cluster (WSFC), with SQL Server keeping more detailed state information about the instances than the cluster cares about. The management model is that SQL Server must drive the transactions, and is responsible for keeping the cluster's view of the state in sync with SQL Server's view of state. If the state of the cluster is changed outside of SQL Server it's possible for the state to get out of sync between WSFC and SQL Server, which might lead to unpredictable behavior.

  For example:

  • Don't change any availability group properties, such as the possible owners.

  • Don't use the Failover Cluster Manager to fail over availability groups. You must use Transact-SQL or SQL Server Management Studio.

• Don't add resources or alter dependencies associated to availability group role. We don't recommend placing any additional resources (including user or third-party) into the availability group role or altering the role dependencies as these changes can negatively impact failover performance.

Prerequisites (availability groups)

When creating or reconfiguring an availability group configuration, ensure that you adhere to the following requirements.

Security (availability groups)

• Security is inherited from the WSFC. Windows Server failover clustering provides two levels of user security at the granularity of entire cluster:

  • Read-only access

  • Full control

    Always On availability groups need full control, and enabling Always On availability groups on an instance of SQL Server gives it full control of the cluster (through Service SID).

    You can't directly add or remove security for a server instance in Cluster Manager. To manage cluster security sessions, use the SQL Server Configuration Manager or the WMI equivalent from SQL Server.

• Each instance of SQL Server must have permissions to access the registry, cluster, and so forth.

• We recommend that you use encryption for connections between server instances that host Always On availability groups availability replicas.

Permissions (availability groups)

Related tasks (availability groups)

Availability database prerequisites and restrictions

To be eligible to be added to an availability group, a database must meet the following prerequisites and restrictions.

In this section:

• Requirements
• Restrictions
• Recommendations for computers that host availability replicas (Windows system
• Permissions
• Related Tasks

Checklist: Requirements (availability databases)

To be eligible to be added to an availability group, a database must:

Restrictions (availability databases)

• If the file path (including the drive letter) of a secondary database differs from the path of the corresponding primary database, the following restrictions apply:

  • New Availability Group Wizard/Add Database to Availability Group Wizard: The Full option isn't supported (on the Select Initial Data Synchronization Page (Always On Availability Group Wizards) page),

  • RESTORE WITH MOVE: To create the secondary databases, the database files must be RESTORED WITH MOVE on each instance of SQL Server that hosts a secondary replica.

  • Impact on add-file operations: A later add-file operation on the primary replica might fail on the secondary databases. This failure could cause the secondary databases to be suspended. This, in turn, causes the secondary replicas to enter the NOT SYNCHRONIZING state.

    Note

    For information about responding to a failed ad-file operation, see Troubleshoot a Failed Add-File Operation (Always On Availability Groups).

• You can't drop a database that currently belongs to an availability group.

Follow up for TDE protected databases

If you use transparent data encryption (TDE), the certificate or asymmetric key for creating and decrypting other keys must be the same on every server instance that hosts an availability replica for the availability group. For more information, see Move a TDE Protected Database to Another SQL Server.

Permissions (availability databases)

Requires ALTER permission on the database.

Related tasks (availability databases)

Related content

• Microsoft SQL Server Always On Solutions Guide for High Availability and Disaster Recovery
• SQL Server Always On Team Blog: The official SQL Server Always On Team Blog
• Always On - HADRON Learning Series: Worker Pool Usage for HADRON Enabled Databases
• What is an Always On availability group?
• Failover Clustering and Always On Availability Groups (SQL Server)
• Driver and client connectivity support for availability groups