Operate Red Hat Enterprise Linux shared disk cluster for SQL Server

APPLIES TO: yesSQL Server (Linux only) noAzure SQL Database noAzure SQL Data Warehouse noParallel Data Warehouse

This document describes how to do the following tasks for SQL Server on a shared disk failover cluster with Red Hat Enterprise Linux.

  • Manually failover the cluster
  • Monitor a failover cluster SQL Server service
  • Add a cluster node
  • Remove a cluster node
  • Change the SQL Server resource monitoring frequency

Architecture description

The clustering layer is based on Red Hat Enterprise Linux (RHEL) HA add-on built on top of Pacemaker. Corosync and Pacemaker coordinate cluster communications and resource management. The SQL Server instance is active on either one node or the other.

The following diagram illustrates the components in a Linux cluster with SQL Server.

Red Hat Enterprise Linux 7 Shared Disk SQL Cluster

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

Failover cluster manually

The resource move command creates a constraint forcing the resource to start on the target node.  After executing the move command, executing resource clear will remove the constraint so it is possible to move the resource again or have the resource automatically fail over.

sudo pcs resource move <sqlResourceName> <targetNodeName>  
sudo pcs resource clear <sqlResourceName> 

The following example moves the mssqlha resource to a node named sqlfcivm2, and then removes the constraint so that the resource can move to a different node later.

sudo pcs resource move mssqlha sqlfcivm2 
sudo pcs resource clear mssqlha 

Monitor a failover cluster SQL Server service

View the current cluster status:

sudo pcs status  

View live status of cluster and resources:

sudo crm_mon 

View the resource agent logs at /var/log/cluster/corosync.log

Add a node to a cluster

  1. Check the IP address for each node. The following script shows the IP address of your current node.

    ip addr show
  2. The new node needs a unique name that is 15 characters or less. By default in Red Hat Linux the computer name is localhost.localdomain. This default name may not be unique and is too long. Set the computer name the new node. Set the computer name by adding it to /etc/hosts. The following script lets you edit /etc/hosts with vi.

    sudo vi /etc/hosts

    The following example shows /etc/hosts with additions for three nodes named sqlfcivm1, sqlfcivm2, andsqlfcivm3.   localhost localhost4 localhost4.localdomain4
    ::1         localhost localhost6 localhost6.localdomain6 fcivm1 fcivm2 fcivm3

    The file should be the same on every node.

  3. Stop the SQL Server service on the new node.

  4. Follow the instructions to mount the database file directory to the shared location:

    From the NFS server, install nfs-utils

    sudo yum -y install nfs-utils 

    Open up the firewall on clients and NFS server

    sudo firewall-cmd --permanent --add-service=nfs
    sudo firewall-cmd --permanent --add-service=mountd
    sudo firewall-cmd --permanent --add-service=rpc-bind
    sudo firewall-cmd --reload

    Edit /etc/fstab file to include the mount command:

    <IP OF NFS SERVER>:<shared_storage_path> <database_files_directory_path> nfs timeo=14,intr

    Run mount -a for the changes to take effect.

  5. On the new node, create a file to store the SQL Server username and password for the Pacemaker login. The following command creates and populates this file:

    sudo touch /var/opt/mssql/passwd
    sudo echo "<loginName>" >> /var/opt/mssql/secrets/passwd
    sudo echo "<loginPassword>" >> /var/opt/mssql/secrets/passwd
    sudo chown root:root /var/opt/mssql/passwd
    sudo chmod 600 /var/opt/mssql/passwd
  6. On the new node, 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 you're using another firewall that doesn't have a built-in high-availability configuration, the following ports need to be opened for Pacemaker to be able to communicate with other nodes in the cluster

    • TCP: Ports 2224, 3121, 21064
    • UDP: Port 5405
  7. Install Pacemaker packages on the new node.

    sudo yum install pacemaker pcs fence-agents-all resource-agents
  8. Set the password for the default user that is created when installing Pacemaker and Corosync packages. Use the same password as the existing nodes.

    sudo passwd hacluster
  9. Enable and start pcsd service and Pacemaker. This will allow the new node to rejoin the cluster after the reboot. Run the following command on the new node.

    sudo systemctl enable pcsd
    sudo systemctl start pcsd
    sudo systemctl enable pacemaker
  10. Install the FCI resource agent for SQL Server. Run the following commands on the new node.

    sudo yum install mssql-server-ha
  11. On an existing node from the cluster, authenticate the new node and add it to the cluster:

    sudo pcs    cluster auth <nodeName3> -u hacluster 
    sudo pcs    cluster node add <nodeName3> 

    The following example adds a node named vm3 to the cluster.

    sudo pcs    cluster auth  
    sudo pcs    cluster start 

Remove nodes from a cluster

To remove a node from a cluster run the following command:

sudo pcs    cluster node remove <nodeName>  

Change the frequency of sqlservr resource monitoring interval

sudo pcs    resource op monitor interval=<interval>s <sqlResourceName> 

The following example sets the monitoring interval to 2 seconds for the mssql resource:

sudo pcs    resource op monitor interval=2s mssqlha 

Troubleshoot Red Hat Enterprise Linux shared disk cluster for SQL Server

In troubleshooting the cluster it may help to understand how the three daemons work together to manage cluster resources.

Daemon Description
Corosync Provides quorum membership and messaging between cluster nodes.
Pacemaker Resides on top of Corosync and provides state machines for resources.
PCSD Manages both Pacemaker and Corosync through the pcs tools

PCSD must be running in order to use pcs tools.

Current cluster status

sudo pcs status returns basic information about the cluster, quorum, nodes, resources, and daemon status for each node.

An example of a healthy pacemaker quorum output would be:

Cluster name: MyAppSQL 
Last updated: Wed Oct 31 12:00:00 2016  Last change: Wed Oct 31 11:00:00 2016 by root via crm_resource on sqlvmnode1 
Stack: corosync 
Current DC: sqlvmnode1  (version 1.1.13-10.el7_2.4-44eb2dd) - partition with quorum 
3 nodes and 1 resource configured 

Online: [ sqlvmnode1 sqlvmnode2 sqlvmnode3] 

Full list of resources: 

mssqlha (ocf::sql:fci): Started sqlvmnode1 

PCSD Status: 
sqlvmnode1: Online 
sqlvmnode2: Online 
sqlvmnode3: Online 

Daemon Status: 
corosync: active/disabled 
pacemaker: active/enabled 

In the example, partition with quorum means that a majority quorum of nodes is online. If the cluster loses a majority quorum of nodes , pcs status will return partition WITHOUT quorum and all resources will be stopped.

online: [sqlvmnode1 sqlvmnode2 sqlvmnode3] returns the name of all nodes currently participating in the cluster. If any nodes are not participating, pcs status returns OFFLINE: [<nodename>].

PCSD Status shows the cluster status for each node.

Reasons why a node may be offline

Check the following items when a node is offline.

  • Firewall

    The following ports need to be open on all nodes for Pacemaker to be able to communicate.

    • **TCP: 2224, 3121, 21064
  • Pacemaker or Corosync services running

  • Node communication

  • Node name mappings

Additional resources

Next steps

Configure Red Hat Enterprise Linux shared disk cluster for SQL Server