Configure SLES Cluster for SQL Server Availability Group
This guide provides instructions to create a three-node cluster for SQL Server on SUSE Linux Enterprise Server (SLES) 12 SP2. 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 SUSE High Availability Extension (HAE) built on top of Pacemaker.
For more information on cluster configuration, resource agent options, management, best practices, and recommendations, see SUSE Linux Enterprise High Availability Extension 12 SP2.
At this point, SQL Server's integration with Pacemaker on Linux is not as coupled as with WSFC on Windows. SQL Server service on Linux is not cluster aware. Pacemaker controls all of the orchestration of the cluster resources, including the availability group resource. On Linux, you should not rely on Always On Availability Group Dynamic Management Views (DMVs) that provide cluster information like sys.dm_hadr_cluster. Also, virtual network name is specific to WSFC, there is no equivalent of the same in Pacemaker. You can still create a listener to use it for transparent reconnection after failover, but you will have to manually register the listener name in the DNS server with the IP used to create the virtual IP resource (as explained in the following sections).
The procedure for creating an availability group for high availability differs between Linux servers and a Windows Server failover cluster. The following list describes the high-level steps:
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.
Production environments require a fencing agent, like STONITH for high availability. The examples in this article do not use fencing agents. They are for testing and validation only.
A Pacemaker cluster uses fencing to return the cluster to a known state. The way to configure fencing depends on the distribution and the environment. At this time, fencing is not available in some cloud environments. See SUSE Linux Enterprise High Availability Extension.
To complete the following end-to-end scenario, you need three machines to deploy the three nodes cluster. The following steps outline how to configure these servers.
Setup and configure the operating system on each cluster node
The first step is to configure the operating system on the cluster nodes. For this walk through, use SLES 12 SP2 with a valid subscription for the HA add-on.
Install and configure SQL Server service on each cluster node
Install and setup SQL Server service on all nodes. For detailed instructions, see Install SQL Server on Linux.
Designate one node as primary and other nodes as secondaries. Use these terms throughout this guide.
Make sure nodes that are going to be part of the cluster can communicate with each other.
The following example shows
/etc/hostswith additions for three nodes named SLES1, SLES2, and SLES3.
127.0.0.1 localhost 10.128.16.33 SLES1 10.128.16.77 SLES2 10.128.16.22 SLES3
All cluster nodes must be able to access each other via SSH. Tools like
crm_report(for troubleshooting) and Hawk's History Explorer require passwordless SSH access between the nodes, otherwise they can only collect data from the current node. In case you use a non-standard SSH port, use the -X option (see
manpage). For example, if your SSH port is 3479, invoke a
sudo crm_report -X "-p 3479" [...]
For more information, see the SLES Administration Guide - Miscellaneous section.
Create a SQL Server login for Pacemaker
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@$$w0rd!' ALTER SERVER ROLE [sysadmin] ADD MEMBER [pacemakerLogin]
At the time of availability group creation, the pacemaker user will require ALTER, CONTROL and VIEW DEFINITION permissions on the availability group, after it's created but before any nodes are added to it.
On all SQL Servers, save the credentials for the SQL Server login.
echo 'pacemakerLogin' >> ~/pacemaker-passwd echo 'ComplexP@$$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
Configure an Always On Availability Group
On Linux servers, configure the availability group and then configure the cluster resources. To configure the availability group, see Configure Always On Availability Group for SQL Server on Linux
Install and configure Pacemaker on each cluster node
Install the High Availability extension
For reference, see Installing SUSE Linux Enterprise Server and High Availability Extension
Install SQL Server resource agent package on both nodes.
sudo zypper install mssql-server-ha
Set up the first node
Refer to SLES installation instructions
Log in as
rootto the physical or virtual machine you want to use as cluster node.
Start the bootstrap script by executing:
If NTP has not been configured to start at boot time, a message appears.
If you decide to continue anyway, the script automatically generates keys for SSH access and for the Csync2 synchronization tool and start the services needed for both.
To configure the cluster communication layer (Corosync):
a. Enter a network address to bind to. By default, the script proposes the network address of eth0. Alternatively, enter a different network address, for example the address of bond0.
b. Enter a multicast address. The script proposes a random address that you can use as default.
c. Enter a multicast port. The script proposes 5405 as default.
d. To configure
SBD (), enter a persistent path to the partition of your block device that you want to use for SBD. The path must be consistent across all nodes in the cluster. Finally, the script will start the Pacemaker service to bring the one-node cluster online and enable the Web management interface Hawk2. The URL to use for Hawk2 is displayed on the screen.
For any details of the setup process, check
/var/log/sleha-bootstrap.log. You now have a running one-node cluster. Check the cluster status with crm status:
sudo crm status
You can also see cluster configuration with
crm configure show xmlor
crm configure show.
The bootstrap procedure creates a Linux user named hacluster with the password linux. Replace the default password with a secure one as soon as possible:
sudo passwd hacluster
Add nodes to the existing cluster
If you have a cluster running with one or more nodes, add more cluster nodes with the ha-cluster-join bootstrap script. The script only needs access to an existing cluster node and will complete the basic setup on the current machine automatically. Use the following steps:
If you have configured the existing cluster nodes with the
YaST cluster module, make sure the following prerequisites are fulfilled before you run
- The root user on the existing nodes has SSH keys in place for passwordless login.
Csync2is configured on the existing nodes. For more information,see Configuring Csync2 with YaST.
Log in as root to the physical or virtual machine supposed to join the cluster.
Start the bootstrap script by executing:
If NTP has not been configured to start at boot time, a message appears.
If you decide to continue anyway, you will be prompted for the IP address of an existing node. Enter the IP address.
If you have not already configured a passwordless SSH access between both machines, you will also be prompted for the root password of the existing node.
After logging in to the specified node, the script copies the Corosync configuration, configures SSH and
Csync2, and brings the current machine online as new cluster node. Apart from that, it starts the service needed for Hawk. If you have configured shared storage with
OCFS2, it also automatically creates the mountpoint directory for the
Repeat the previous steps for all machines you want to add to the cluster.
For details of the process, check
Check the cluster status with
sudo crm status. If you have successfully added a second node, the output will be similar to the following:
sudo crm status 3 nodes configured 1 resource configured Online: [ SLES1 SLES2 SLES3] Full list of resources: admin_addr (ocf::heartbeat:IPaddr2): Started node1
admin_addris the virtual IP cluster resource which is configured during initial one-node cluster setup.
After adding all nodes, check if you need to adjust the no-quorum-policy in the global cluster options. This is especially important for two-node clusters. For more information,see Section 4.1.2, Option no-quorum-policy.
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:
crm configure property cluster-recheck-interval=2min
If you already have an availability group resource managed by a Pacemaker cluster, note that all distributions that use the latest available Pacemaker package 1.1.18-11.el7 introduce a behavior change for the start-failure-is-fatal cluster setting when its value is false. This change affects the failover workflow. If a primary replica experiences an outage, the cluster is expected to failover to one of the available secondary replicas. Instead, users will notice that the cluster keeps trying to start the failed primary replica. If that primary never comes online (because of a permanent outage), the cluster never fails over to another available secondary replica. Because of this change, a previously recommended configuration to set start-failure-is-fatal is no longer valid and the setting needs to be reverted back to its default value of
Additionally, the AG resource needs to be updated to include the
To update the property value to
crm configure property start-failure-is-fatal=true
Update your existing AG resource property
60s run (replace
ag1 with the name of your availability group resource):
crm configure edit ag1 # In the text editor, add `meta failure-timeout=60s` after any `param`s and before any `op`s
For more information on Pacemaker cluster properties, see Configuring Cluster Resources.
Configure fencing (STONITH)
Pacemaker cluster vendors require STONITH to be enabled and a fencing device configured for a supported cluster setup. When the cluster resource manager cannot determine the state of a node or of a resource on a node, fencing is used to bring the cluster to a known state again.
Resource level fencing ensures mainly that there is no data corruption during an outage by configuring a resource. You can use resource level fencing, for instance, with DRBD (Distributed Replicated Block Device) 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 and the Pacemaker implementation of it is called STONITH (which stands for "shoot the other node in the head"). Pacemaker supports a great variety of fencing devices, such as an uninterruptible power supply or management interface cards for servers.
At cluster initialization time, STONITH is disabled if no configuration is detected. It can be enabled later by running following command:
sudo crm configure property stonith-enabled=true
Disabling STONITH is just for testing purposes. If you plan to use Pacemaker in a production environment, you should plan a STONITH implementation depending on your environment and keep it enabled. SUSE does not provide fencing agents for any cloud environments (including Azure) or Hyper-V. Consequentially, the cluster vendor does not offer support for running production clusters in these environments. We are working on a solution for this gap that will be available in future releases.
Configure the cluster resources for SQL Server
Refer to SLES Administration Guid
Enable Pacemaker so that it automatically starts.
Run the following command on every node in the cluster.
systemctl enable pacemaker
Create availability group resource
The following command creates and configures the availability group resource for three replicas of availability group [ag1]. The monitor operations and timeouts have to be specified explicitly in SLES based on the fact that timeouts are highly workload-dependent and need to be carefully adjusted for each deployment. Run the command on one of the nodes in the cluster:
crm configureto open the crm prompt:
sudo crm configure
In the crm prompt, run the following command to configure the resource properties.
primitive ag_cluster \ ocf:mssql:ag \ params ag_name="ag1" \ meta failure-timeout=60s \ op start timeout=60s \ op stop timeout=60s \ op promote timeout=60s \ op demote timeout=10s \ op monitor timeout=60s interval=10s \ op monitor timeout=60s interval=11s role="Master" \ op monitor timeout=60s interval=12s role="Slave" \ op notify timeout=60s ms ms-ag_cluster ag_cluster \ meta master-max="1" master-node-max="1" clone-max="3" \ clone-node-max="1" notify="true" \ commit
When you create the resource, and periodically afterwards, the Pacemaker resource agent automatically sets the value of
REQUIRED_SYNCHRONIZED_SECONDARIES_TO_COMMIT on the availability group based on the availability group's configuration. For example, if the availability group has three synchronous replicas, the agent will set
1. For details and additional configuration options, see High availability and data protection for availability group configurations.
Create virtual IP resource
If you did not create the virtual IP resource when you ran
ha-cluster-init you can create this resource now. The following command creates a virtual IP resource. Replace
<**0.0.0.0**> with an available address from your network and
<**24**> with the number of bits in the CIDR subnet mask. Run on one node.
crm configure \ primitive admin_addr \ ocf:heartbeat:IPaddr2 \ params ip=<**0.0.0.0**> \ cidr_netmask=<**24**>
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, and 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.) We can manipulate the decisions of the cluster with constraints. Constraints have a score. If a constraint has a score lower than INFINITY, it is only a recommendation. A score of INFINITY means it is a must. We want to ensure that primary of the availability group and the virtual ip resource are run on the same host, so we define a colocation constraint with a score of INFINITY.
To set colocation constraint for the virtual IP to run on same node as the master, run the following command on one node:
crm configure colocation vip_on_master inf: \ admin_addr ms-ag_cluster:Master commit
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:
- User issues resource migrate to the availability group master from node1 to node2.
- The virtual IP resource stops on node 1.
- The virtual IP resource starts on node 2. At this point, the IP address temporarily points to node 2 while node 2 is still a pre-failover secondary.
- The availability group master on node 1 is demoted to slave.
- The availability group slave on node 2 is promoted to master.
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:
crm crm configure \ order ag_first inf: ms-ag_cluster:promote admin_addr:start
After you configure the cluster and add the availability group as a cluster resource, you cannot use Transact-SQL to fail over the availability group resources. SQL Server cluster resources on Linux are not coupled as tightly with the operating system as they are on a Windows Server Failover Cluster (WSFC). SQL Server service is not aware of the presence of the cluster. All orchestration is done through the cluster management tools. In SLES use
Manually fail over the availability group with
crm. Don't initiate failover with Transact-SQL. For more information, see Failover.
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