Configure RHEL Cluster for SQL Server Availability Group

• 01/23/2020
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Applies to: SQL Server (all supported versions) - Linux

This document explains how to create a three-node availability group cluster for SQL Server on Red Hat Enterprise Linux. For high availability, an availability group on Linux requires three nodes - see High availability and data protection for availability group configurations. The clustering layer is based on Red Hat Enterprise Linux (RHEL) HA add-on built on top of Pacemaker.

For more information on cluster configuration, resource agents options, and management, visit RHEL reference documentation.

The following sections walk through the steps to set up a Pacemaker cluster and add an availability group as resource in the cluster for high availability.

Roadmap

The steps to create an availability group on Linux servers for high availability are different from the steps on a Windows Server failover cluster. The following list describes the high-level steps:

1. Configure SQL Server on the cluster nodes.

2. Create the availability group.

3. Configure a cluster resource manager, like Pacemaker. These instructions are in this document.

   The way to configure a cluster resource manager depends on the specific Linux distribution.

   Important

   Production environments require a fencing agent, like STONITH for high availability. The demonstrations in this documentation do not use fencing agents. The demonstrations are for testing and validation only.

   A Linux cluster uses fencing to return the cluster to a known state. The way to configure fencing depends on the distribution and the environment. Currently, fencing is not available in some cloud environments. For more information, see Support Policies for RHEL High Availability Clusters - Virtualization Platforms.

4. Add the availability group as a resource in the cluster.

Configure high availability for RHEL

To configure high availability for RHEL, enable the high availability subscription and then configure Pacemaker.

Enable the high availability subscription for RHEL

Each node in the cluster must have an appropriate subscription for RHEL and the High Availability Add on. Review the requirements at How to install High Availability cluster packages in Red Hat Enterprise Linux. Follow these steps to configure the subscription and repos:

1. Register the system.

   sudo subscription-manager register
   

   Provide your user name and password.

2. List the available pools for registration.

   sudo subscription-manager list --available
   

   From the list of available pools, note the pool ID for the high availability subscription.

3. Update the following script. Replace <pool id> with the pool ID for high availability from the preceding step. Run the script to attach the subscription.

   sudo subscription-manager attach --pool=<pool id>
   

4. Enable the repository.

   RHEL 7

   sudo subscription-manager repos --enable=rhel-ha-for-rhel-7-server-rpms
   

   RHEL 8

   sudo subscription-manager repos --enable=rhel-8-for-x86_64-highavailability-rpms
   

For more information, see Pacemaker - The Open Source, High Availability Cluster.

After you have configured the subscription, complete the following steps to configure Pacemaker:

Configure Pacemaker

After you register the subscription, complete the following steps to configure Pacemaker:

1. On all cluster nodes, open the Pacemaker firewall ports. To open these ports with firewalld, run the following command:

   sudo firewall-cmd --permanent --add-service=high-availability
   sudo firewall-cmd --reload
   

   If the firewall doesn't have a built-in high-availability configuration, open the following ports for Pacemaker.

   • TCP: Ports 2224, 3121, 21064
   • UDP: Port 5405

2. Install Pacemaker packages on all nodes.

   sudo yum install pacemaker pcs fence-agents-all resource-agents
   

3. Set the password for the default user that is created when installing Pacemaker and Corosync packages. Use the same password on all nodes.

   sudo passwd hacluster
   

4. To allow nodes to rejoin the cluster after the reboot, enable and start pcsd service and Pacemaker. Run the following command on all nodes.

   sudo systemctl enable pcsd
   sudo systemctl start pcsd
   sudo systemctl enable pacemaker
   

5. Create the Cluster. To create the cluster, run the following command:

   RHEL 7

   sudo pcs cluster auth <node1> <node2> <node3> -u hacluster -p <password for hacluster>
   sudo pcs cluster setup --name <clusterName> <node1> <node2> <node3> 
   sudo pcs cluster start --all
   sudo pcs cluster enable --all
   

   RHEL8

   For RHEL 8, you will need to authenticate the nodes separately. Manually enter in the Username and Password for hacluster when prompted.

   sudo pcs host auth <node1> <node2> <node3>
   sudo pcs cluster setup <clusterName> <node1> <node2> <node3>
   sudo pcs cluster start --all
   sudo pcs cluster enable --all
   

   Note

   If you previously configured a cluster on the same nodes, you need to use --force option when running pcs cluster setup. This option is equivalent to running pcs cluster destroy. To re-enable pacemaker, run sudo systemctl enable pacemaker.

6. Install SQL Server resource agent for SQL Server. Run the following commands on all nodes.

   sudo yum install mssql-server-ha
   

After Pacemaker is configured, use pcs to interact with the cluster. Execute all commands on one node from the cluster.

Configure fencing (STONITH)

Pacemaker cluster vendors require STONITH to be enabled and a fencing device configured for a supported cluster setup. STONITH stands for "shoot the other node in the head." When the cluster resource manager cannot determine the state of a node or of a resource on a node, fencing brings the cluster to a known state again.

A STONITH device provides a fencing agent. Setting up Pacemaker on Red Hat Enterprise Linux in Azure provides an example of how to create a STONITH device for this cluster in Azure. Modify the instructions for your environment.

Resource level fencing ensures that there is no data corruption in case of an outage by configuring a resource. For example, you can use resource level fencing to mark the disk on a node as outdated when the communication link goes down.

Node level fencing ensures that a node does not run any resources. This is done by resetting the node. Pacemaker supports a great variety of fencing devices. Examples include an uninterruptible power supply or management interface cards for servers.

For information about STONITH, and fencing, see the following articles:

• Pacemaker Clusters from Scratch
• Fencing and STONITH
• Red Hat High Availability Add-On with Pacemaker: Fencing

sudo pcs property set stonith-enabled=false

Set cluster property cluster-recheck-interval

cluster-recheck-interval indicates the polling interval at which the cluster checks for changes in the resource parameters, constraints or other cluster options. If a replica goes down, the cluster tries to restart the replica at an interval that is bound by the failure-timeout value and the cluster-recheck-interval value. For example, if failure-timeout is set to 60 seconds and cluster-recheck-interval is set to 120 seconds, the restart is tried at an interval that is greater than 60 seconds but less than 120 seconds. We recommend that you set failure-timeout to 60s and cluster-recheck-interval to a value that is greater than 60 seconds. Setting cluster-recheck-interval to a small value is not recommended.

To update the property value to 2 minutes run:

sudo pcs property set cluster-recheck-interval=2min

To update the property value to true run:

sudo pcs property set start-failure-is-fatal=true

To update the ag_cluster resource property failure-timeout to 60s run:

pcs resource update ag_cluster meta failure-timeout=60s

For information on Pacemaker cluster properties, see Pacemaker Clusters Properties.

Create a SQL Server login for Pacemaker

1. On all SQL Servers, create a Server login for Pacemaker. The following Transact-SQL creates a login:

   USE [master]
   GO
   CREATE LOGIN [pacemakerLogin] with PASSWORD= N'ComplexP@$$w0rd!'
   
   ALTER SERVER ROLE [sysadmin] ADD MEMBER [pacemakerLogin]
   

At the time of availability group creation, the pacemaker user will require ALTER, CONTROL and VIEW DEFINITION permissions on the availability group, after it's created but before any nodes are added to it.

1. On all SQL Servers, save the credentials for the SQL Server login.

   echo 'pacemakerLogin' >> ~/pacemaker-passwd
   echo 'ComplexP@$$w0rd!' >> ~/pacemaker-passwd
   sudo mv ~/pacemaker-passwd /var/opt/mssql/secrets/passwd
   sudo chown root:root /var/opt/mssql/secrets/passwd
   sudo chmod 400 /var/opt/mssql/secrets/passwd # Only readable by root
   

Create availability group resource

To create the availability group resource, use pcs resource create command and set the resource properties. The following command creates a ocf:mssql:ag master/subordinate type resource for availability group with name ag1.

RHEL 7

sudo pcs resource create ag_cluster ocf:mssql:ag ag_name=ag1 meta failure-timeout=60s master notify=true

RHEL 8

With the availability of RHEL 8, the create syntax has changed. If you are using RHEL 8, the terminology master has changed to promotable. Use the following create command instead of the above command:

sudo pcs resource create ag_cluster ocf:mssql:ag ag_name=ag1 meta failure-timeout=60s promotable notify=true

Create virtual IP resource

To create the virtual IP address resource, run the following command on one node. Use an available static IP address from the network. Replace the IP address between <10.128.16.240> with a valid IP address.

sudo pcs resource create virtualip ocf:heartbeat:IPaddr2 ip=<10.128.16.240>

There is no virtual server name equivalent in Pacemaker. To use a connection string that points to a string server name instead of an IP address, register the virtual IP resource address and desired virtual server name in DNS. For DR configurations, register the desired virtual server name and IP address with the DNS servers on both primary and DR site.

Add colocation constraint

Almost every decision in a Pacemaker cluster, like choosing where a resource should run, is done by comparing scores. Scores are calculated per resource. The cluster resource manager chooses the node with the highest score for a particular resource. If a node has a negative score for a resource, the resource cannot run on that node.

On a pacemaker cluster, you can manipulate the decisions of the cluster with constraints. Constraints have a score. If a constraint has a score lower than INFINITY, Pacemaker regards it as recommendation. A score of INFINITY is mandatory.

To ensure that primary replica and the virtual ip resources run on the same host, define a colocation constraint with a score of INFINITY. To add the colocation constraint, run the following command on one node.

RHEL 7

When you create the ag_cluster resource in RHEL 7, it creates the resource as ag_cluster-master. Use the following command for RHEL 7:

sudo pcs constraint colocation add virtualip ag_cluster-master INFINITY with-rsc-role=Master

RHEL 8

When you create the ag_cluster resource in RHEL 8, it creates the resource as ag_cluster-clone. Use the following command for RHEL 8:

sudo pcs constraint colocation add virtualip with master ag_cluster-clone INFINITY with-rsc-role=Master

Add ordering constraint

The colocation constraint has an implicit ordering constraint. It moves the virtual IP resource before it moves the availability group resource. By default the sequence of events is:

1. User issues pcs resource move to the availability group primary from node1 to node2.

2. The virtual IP resource stops on node 1.

3. The virtual IP resource starts on node 2.

   Note

   At this point, the IP address temporarily points to node 2 while node 2 is still a pre-failover secondary.

4. The availability group primary on node 1 is demoted to secondary.

5. The availability group secondary on node 2 is promoted to primary.

To prevent the IP address from temporarily pointing to the node with the pre-failover secondary, add an ordering constraint.

To add an ordering constraint, run the following command on one node:

RHEL 7

sudo pcs constraint order promote ag_cluster-master then start virtualip

RHEL 8

sudo pcs constraint order promote ag_cluster-clone then start virtualip

Manually fail over the availability group with pcs. Do not initiate failover with Transact-SQL. For instructions, see Failover.

Next steps

Operate HA availability group