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Always On availability group failover on Linux

  • Άρθρο

Applies to: SQL Server - Linux

Within the context of an availability group (AG), the primary role and secondary role of availability replicas are typically interchangeable, in a process known as failover. Three forms of failover exist: automatic failover (without data loss), planned manual failover (without data loss), and forced manual failover (with possible data loss), typically called forced failover. Automatic and planned manual failovers preserve all your data. An AG fails over at the availability-replica level. That is, an AG fails over to one of its secondary replicas (the current failover target).

For background information about failover, see Failover and Failover Modes (Always On Availability Groups).

Manual failover

Use the cluster management tools to fail over an AG managed by an external cluster manager. For example, if a solution uses Pacemaker to manage a Linux cluster, use pcs to perform manual failovers on Red Hat Enterprise Linux (RHEL) or Ubuntu. On SUSE Linux Enterprise Server (SLES), use crm.

Important

Under normal operations, don't fail over with Transact-SQL or SQL Server management tools like SSMS or PowerShell. When CLUSTER_TYPE = EXTERNAL, the only acceptable value for FAILOVER_MODE is EXTERNAL. With these settings, all manual or automatic failover actions are executed by the external cluster manager. For instructions to force failover with potential data loss, see Force failover.

Manual failover steps

To fail over, the secondary replica that will become the primary replica must be synchronous. If a secondary replica is asynchronous, change the availability mode.

Manually fail over in two steps.

  1. First, manually fail over by moving AG resource from the cluster node that owns the resources to a new node.

    The cluster fails the AG resource over and adds a location constraint. This constraint configures the resource to run on the new node. Remove this constraint in order to successfully fail over in the future.

  2. Second, remove the location constraint.

Step 1. Manually fail over by moving availability group resource

To manually fail over an AG resource named ag_cluster to cluster node named nodeName2, run the appropriate command for your distribution:

  • RHEL/Ubuntu example

    sudo pcs resource move ag_cluster-master nodeName2 --master --lifetime=30S

  • SLES example

    crm resource migrate ag_cluster nodeName2 --lifetime=30S

When you use the --lifetime option, the location constraint created to move the resource is temporary in nature and is valid for 30 seconds in previous example.

The temporary constraint isn't cleared automatically and might show up in the constraint list, but as an expired constraint. Expired constraints don't affect the failover behavior of pacemaker cluster. If you don't use the --lifetime option when moving the resource, you should remove a location constraint that is automatically added, which is noted in the following section.

Step 2. Remove the location constraint

During a manual failover, the pcs command move or crm command migrate adds a location constraint for the resource to be placed on the new target node. To see the new constraint, run the following command after manually moving the resource:

  • RHEL/Ubuntu example

    sudo pcs constraint list --full

  • SLES example

    crm config show

    This is an example of the constraint, which gets created because of a manual failover.

    Enabled on: Node1 (score:INFINITY) (role: Master) (id:cli-prefer-ag_cluster-master)

    Note

    The AG resource name in pacemaker clusters on Red Hat Enterprise Linux 8.x and Ubuntu 18.04 might resemble ag_cluster-clone as the nomenclature regarding resources has been evolving to use promotable clone.

  • RHEL/Ubuntu example

    In the following command, cli-prefer-ag_cluster-master is the ID of the constraint that needs to be removed. sudo pcs constraint list --full returns this ID.

    sudo pcs resource clear ag_cluster-master

    Or

    sudo pcs constraint remove cli-prefer-ag_cluster-master

    Alternatively, you can perform both move and clearing of auto generated constraints in a single line as follows. The following example uses the clone terminology as per Red Hat Enterprise Linux 8.x.

    sudo pcs resource move ag_cluster-clone --master nodeName2 && sleep 30 && sudo pcs resource clear ag_cluster-clone

  • SLES example

    In the following command, cli-prefer-ms-ag_cluster is the ID of the constraint. crm config show returns this ID.

    crm configure
delete cli-prefer-ms-ag_cluster
commit

Note

Automatic failover doesn't add a location constraint, so no cleanup is necessary.

For more information:

Force failover

A forced failover is intended strictly for disaster recovery. In this case, you can't fail over with cluster management tools because the primary datacenter is down. If you force failover to an unsynchronized secondary replica, some data loss is possible. Only force failover if you must restore service to the AG immediately and are willing to risk losing data.

If you can't use the cluster management tools for interacting with the cluster (for example, if the cluster is unresponsive due to a disaster event in the primary data center), you might have to force failover to bypass the external cluster manager. This procedure isn't recommended for regular operations because it risks data loss. Use it when the cluster management tools fail to execute the failover action. Functionally, this procedure is similar to performing a forced manual failover on an AG in Windows.

This process for forcing failover is specific to SQL Server on Linux.

  1. Verify that the cluster no longer manages the AG resource.

    • Set the resource to unmanaged mode on the target cluster node. This command signals the resource agent to stop resource monitoring and management. For example:

      sudo pcs resource unmanage <resourceName>

    • If the attempt to set the resource mode to unmanaged mode fails, delete the resource. For example:

      sudo pcs resource delete <resourceName>

      Note

      When you delete a resource, it also deletes all of the associated constraints.

  2. On the instance of SQL Server that hosts the secondary replica, set the session context variable external_cluster.

    EXEC sp_set_session_context @key = N'external_cluster', @value = N'yes';

  3. Fail over the AG with Transact-SQL. In the following example, replace <MyAg> with the name of your AG. Connect to the instance of SQL Server that hosts the target secondary replica and run the following command:

    ALTER AVAILABILITY GROUP <MyAg> FORCE_FAILOVER_ALLOW_DATA_LOSS;

  4. After a forced failover, bring the AG to a healthy state before either restarting the cluster resource monitoring and management or recreating the AG resource. Review the Essential Tasks After a Forced Failover.

  5. Either restart cluster resource monitoring and management:

    To restart the cluster resource monitoring and management, run the following command:

    sudo pcs resource manage <resourceName>
sudo pcs resource cleanup <resourceName>

    If you deleted the cluster resource, recreate it. To recreate the cluster resource, follow the instructions at Create availability group resource.

Important

Don't use the preceding steps for disaster recovery drills because they risk data loss. Instead change the asynchronous replica to synchronous, and the instructions for normal manual failover.

Database level monitoring and failover trigger

For CLUSTER_TYPE=EXTERNAL, the failover trigger semantics are different compared to WSFC. When the AG is on an instance of SQL Server in a WSFC, transitioning out of ONLINE state for the database causes the AG health to report a fault. In response, the cluster manager triggers a failover action. On Linux, the SQL Server instance can't communicate with the cluster. Monitoring for database health is done outside-in. If user opted in for database level failover monitoring and failover (by setting the option DB_FAILOVER=ON when creating the AG), the cluster checks if the database state is ONLINE every time it runs a monitoring action. The cluster queries the state in sys.databases. For any state different to ONLINE, it triggers a failover automatically (if automatic failover conditions are met). The actual time of the failover depends on the frequency of the monitoring action, and the database state being updated in sys.databases.

Automatic failover requires at least one synchronous replica.

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