Does Always On require a SQL Cluster ?

Veena Srinivas 251 Reputation points
2021-02-04T14:11:58.143+00:00

Hello,

Does Always ON configuration require both a SQL Instance Level Cluster and a WSFC ?

Or does it require only WSFC ?

Please clarify.

Thanks,
V

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  1. Shashank Singh 6,246 Reputation points
    2021-02-05T10:33:04.963+00:00

    Does Always ON configuration require both a SQL Instance Level Cluster and a WSFC ?

    An Availability group neither requires SQL Server Failover cluster instance nor WSFC as MUST to configure it.

    If you are talking about plain traditional AG, then yes it needs WSFC. But you should also know that starting from SQL Server 2017 you have Clusterless Availability group. So if someone is taking from SQL Server 2012 to 2016 WSFC is MUST for AG but starting from SQL Server 2017 like I said you can have AG without WSFC.

    Regarding SQL FCI it is NOT a requirement at all.

    Hope that clear things

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  1. AmeliaGu-MSFT 13,961 Reputation points Microsoft Vendor
    2021-02-05T03:00:00.79+00:00

    Hi @Veena Srinivas ,

    Yes, you can. An availability replica can be hosted by an FCI instance.
    Deploying Always On availability groups requires a Windows Server Failover Cluster. And you can set up a second layer of failover at the server-instance level by implementing SQL Server an FCI together with the WSFC.

    And there are some restrictions for using a SQL Server Failover Cluster Instance to host an availability replica:

    • 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 cannot host another replica 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.
    • FCIs do not support automatic failover by availability groups: FCIs do not 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 will need to remove the replica from its availability group and then add the replica back into the availability group.

    Please refer to SQL Server Failover Cluster Instances (FCIs) and Availability Groups and Considerations for hosting an Availability Replica on an FCI for more details.
    Best Regards,
    Amelia

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