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Version: v23.0

Quorum Considerations for SQL Server Availability Groups

Applies to:

  • Microsoft SQL Server 2017 CU1 and newer

Summary​

To ensure automatic failover in SQL Server availability group deployments, it's essential to consider architectural requirements related to quorum management. This document outlines the necessary configurations for the SQL Server Availability Group, as well as cluster-level quorum and arbitration.

Configuration Requirements​

For availability groups with an External cluster type, such as those managed by DxEnterprise, two types of quorum and arbitration mechanisms are in effect:

Automatic Failover for High Availability with SQL Server​

To meet Microsoft's requirements for automatic failover with an External cluster type, an availability group must have one of the following configurations:

  • Three synchronous replicas
  • Two synchronous replicas plus a configuration-only replica
note

A configuration-only replica can be hosted on any edition of SQL Server, including SQL Server Express. For more information on AG quorum requirements and SQL Server edition features, refer to Microsoft's documentation.

Cluster-Level Quorum​

In the event of network isolation, a cluster will split into multiple sub-groups, each unaware of the others. In this situation, it is possible for the cluster to go into a split-brain state, where multiple primary replicas are promoted simultaneously. To avoid split-brain, some form of arbitration must be used to ensure that only one group of nodes is allowed to promote and maintain a primary replica. With DxEnterprise, this can be done by configuring a node-majority quorum. By default, DxEnterprise has cluster-level quorum disabled (quorum size set to "0"). To enable node-majority quorum in DxEnterprise, run the following dxcli command:

dxcli set-globalsetting quorum.size 51%

This setting requires more than half of all configured cluster members to be available for applications, such as a SQL Server Availability Group, to start. If a group forms with fewer than 51% of configured cluster members, applications will be shut down to avoid conflicts with a group that has a majority of available cluster members.

note

The quorum.size setting can be configured in three ways:

  • As a percentage of configured cluster members: 51%. This requires at least 51% of configured cluster members to be available for quorum, which represents a simple majority, and is the supported setting for SQL Server Availability groups.
  • As a count of available members: 2. This requires at least two members to be available for quorum.
  • As a count of unavailable members: -2. This tolerates two or fewer unavailable members for quorum.

References​