This describes the characteristics of Always On Availability Groups (AGs) under Linux-based SQL Server installations. It also covers differences between Linux- and Windows Server failover cluster (WSFC)-based AGs.
From a high-level standpoint, availability groups under SQL Server on Linux are the same as they are on WSFC-based implementations. That means that all the limitations and features are the same, with some exceptions. The main differences include:
- Microsoft Distributed Transaction Coordinator (DTC) is supported under Linux starting with SQL Server 2017 CU16. However, DTC is not yet supported on Availability Groups on Linux. If your applications require the use of distributed transactions and need an AG, deploy SQL Server on Windows.
- Linux-based deployments that require high availability use Pacemaker for clustering instead of a WSFC.
- Unlike most configurations for AGs on Windows except for the Workgroup Cluster scenario, Pacemaker never requires Active Directory Domain Services (AD DS).
- How to fail an AG from one node to another is different between Linux and Windows.
- Certain settings such as required_synchronized_secondaries_to_commit can only be changed via Pacemaker on Linux, whereas a WSFC-based install uses Transact-SQL.
Number of replicas and cluster nodes
An AG in SQL Server Standard can have two total replicas: one primary, and one secondary that can only be used for availability purposes. It cannot be used for anything else, such as readable queries. An AG in SQL Server Enterprise can have up to nine total replicas: one primary and up to eight secondaries, of which up to three (including the primary) can be synchronous. If using an underlying cluster, there can be a maximum of 16 nodes total when Corosync is involved. An availability group can span at most nine of the 16 nodes with SQL Server Enterprise, and two with SQL Server Standard.
A two-replica configuration that requires the ability to automatically fail over to another replica requires the use of a configuration-only replica. Configuration-only replicas were introduced in SQL Server 2017 (14.x) Cumulative Update 1 (CU1), so that should be the minimum version deployed for this configuration.
If Pacemaker is used, it must be properly configured so it remains up and running. That means that quorum and STONITH must be implemented properly from a Pacemaker perspective, in addition to any SQL Server requirements such as a configuration-only replica.
Readable secondary replicas are only supported with SQL Server Enterprise.
Cluster type and failover mode
New to SQL Server 2017 (14.x) is the introduction of a cluster type for AGs. For Linux, there are two valid values: External and None. A cluster type of External means that Pacemaker will be used underneath the AG. Using External for cluster type requires that the failover mode be set to External as well (also new in SQL Server 2017 (14.x)). Automatic failover is supported, but unlike a WSFC, failover mode is set to External, not automatic, when Pacemaker is used. Unlike a WSFC, the Pacemaker portion of the AG is created after the AG is configured.
A cluster type of None means that there is no requirement for, nor will the AG use, Pacemaker. Even on servers that have Pacemaker configured, if an AG is configured with a cluster type of None, Pacemaker will not see or manage that AG. A cluster type of None only supports manual failover from a primary to a secondary replica. An AG created with None is primarily targeted for the read-scale out scenario as well as upgrades. While it can work in scenarios like disaster recovery or local availability where no automatic failover is necessary, it is not recommended. The listener story is also more complex without Pacemaker.
Cluster type is stored in the SQL Server dynamic management view (DMV) sys.availability_groups, in the columns cluster_type and cluster_type_desc.
New to SQL Server 2017 (14.x) is a setting that is used by AGs called required_synchronized_secondaries_to_commit. This tells the AG the number of secondary replicas that must be in lockstep with the primary. This enables things like automatic failover (only when integrated with Pacemaker with a cluster type of External), and controls the behavior of things like the availability of the primary if the right number of secondary replicas is either online or offline.
The required_synchronized_secondaries_to_commit value is set by default and maintained by Pacemaker/ SQL Server. You can manually override this value.
The combination of required_synchronized_secondaries_to_commit and the new sequence number (which is stored in sys.availability_groups) informs Pacemaker and SQL Server that, for example, automatic failover can happen. In that case, a secondary replica would have the same sequence number as the primary, meaning it is up to date with all the latest configuration information.
There are three values that can be set for required_synchronized_secondaries_to_commit: 0, 1, or 2. They control the behavior of what happens when a replica becomes unavailable. The numbers correspond to the number of secondary replicas that must be synchronized with the primary. The behavior is as follows under Linux:
- 0 – Secondary replicas do not need to be in synchronized state with the primary. However if the secondaries are not synchronized, there will be no automatic failover.
- 1 – One secondary replica must be in a synchronized state with the primary; automatic failover is possible. The primary database is unavailable until a secondary synchronous replica is available.
- 2 – Both secondary replicas in a three or more node AG configuration must be synchronized with the primary; automatic failover is possible.
required_synchronized_secondaries_to_commit controls not only the behavior of failovers with synchronous replicas, but data loss. With a value of 1 or 2, a secondary replica is always required to be synchronized, so there will always be data redundancy. That means no data loss.
To change the value of required_synchronized_secondaries_to_commit, use the following syntax:
Red Hat Enterprise Linux (RHEL) and Ubuntu
sudo pcs resource update <AGResourceName> required_synchronized_secondaries_to_commit=<Value>
SUSE Linux Enterprise Server (SLES)
sudo crm resource param ms-<AGResourceName> set required_synchronized_secondaries_to_commit <value>
where AGResourceName is the name of the resource configured for the AG, and Value is 0, 1, or 2. To set it back to the default of Pacemaker managing the parameter, execute the same statement with no value.
Automatic failover of an AG is possible when the following conditions are met:
- The primary and the secondary replica are set to synchronous data movement.
- The secondary has a state of synchronized (not synchronizing), meaning the two are at the same data point.
- The cluster type is set to External. Automatic failover is not possible with a cluster type of None.
- The sequence_number of the secondary replica to become the primary has the highest sequence number – in other words, the secondary replica’s sequence_number matches the one from the original primary replica.
If these conditions are met and the server hosting the primary replica fails, the AG will change ownership to a synchronous replica. The behavior for synchronous replicas (of which there can be three total: one primary and two secondary replicas) can further be controlled by required_synchronized_secondaries_to_commit. This works with AGs on both Windows and Linux, but is configured completely differently. On Linux, the value is configured automatically by the cluster on the AG resource itself.
Configuration-only replica and quorum
Also new in SQL Server 2017 (14.x) as of CU1 is a configuration-only replica. Because Pacemaker is different than a WSFC, especially when it comes to quorum and requiring STONITH, having just a two-node configuration will not work when it comes to an AG. For an FCI, the quorum mechanisms provided by Pacemaker can be fine, because all FCI failover arbitration happens at the cluster layer. For an AG, arbitration under Linux happens in SQL Server, where all the metadata is stored. This is where the configuration-only replica comes into play.
Without anything else, a third node and at least one synchronized replica would be required. The configuration-only replica stores the AG configuration in the master database, same as the other replicas in the AG configuration. The configuration-only replica does not have the user databases participating in the AG. The configuration data is sent synchronously from the primary. This configuration data is then used during failovers, whether they are automatic or manual.
For an AG to maintain quorum and enable automatic failovers with a cluster type of External, it either must:
- Have three synchronous replicas (SQL Server Enterprise only); or
- Have two replicas (primary and secondary) as well as a configuration only replica.
Manual failovers can happen whether using External or None cluster types for AG configurations. While a configuration-only replica can be configured with an AG that has a cluster type of None, it is not recommended, since it complicates the deployment. For those configurations, manually modify required_synchronized_secondaries_to_commit to have a value of at least 1, so that there is at least one synchronized replica.
A configuration-only replica can be hosted on any edition of SQL Server, including SQL Server Express. This will minimize licensing costs and ensures it works with AGs in SQL Server Standard. This means that the third required server just needs to meet the minimum specification for SQL Server, since it is not receiving user transaction traffic for the AG.
When a configuration-only replica is used, it has the following behavior:
- By default, required_synchronized_secondaries_to_commit is set to 0. This can be manually modified to 1 if desired.
- If the primary fails and required_synchronized_secondaries_to_commit is 0, the secondary replica will become the new primary and be available for both reading and writing. If the value is 1, automatic failover will occur, but will not accept new transactions until the other replica is online.
- If a secondary replica fails and required_synchronized_secondaries_to_commit is 0, the primary replica still accepts transactions, but if the primary fails at this point, there is no protection for the data nor failover possible (manual or automatic), since a secondary replica is not available.
- If the configuration-only replicas fails, the AG will function normally, but no automatic failover is possible.
- If both a synchronous secondary replica and the configuration-only replica fail, the primary cannot accept transactions, and there is nowhere for the primary to fail to.
In CU1 there is a known bug in the logging in the corosync.log file that is generated via mssql-server-ha. If a secondary replica is not able to become the primary due to the number of required replicas available, the current message says “Expected to receive 1 sequence numbers but only received 2. Not enough replicas are online to safely promote the local replica.” The numbers should be reversed, and it should say “Expected to receive 2 sequence numbers but only received 1. Not enough replicas are online to safely promote the local replica.”
Multiple availability groups
More than one AG can be created per Pacemaker cluster or set of servers. The only limitation is system resources. AG ownership is shown by the master. Different AGs can be owned by different nodes; they do not all need to be running on the same node.
Drive and folder location for databases
As on Windows-based AGs, the drive and folder structure for the user databases participating in an AG should be identical. For example, if the user databases are in /var/opt/mssql/userdata on Server A, that same folder should exist on Server B.
The listener under Linux
The listener is optional functionality for an AG. It provides a single point of entry for all connections (read/write to the primary replica and/or read-only to secondary replicas) so that applications and end users do not need to know which server is hosting the data. In a WSFC, this is the combination of a network name resource and an IP resource, which is then registered in AD DS (if needed) as well as DNS. In combination with the AG resource itself, it provides that abstraction.
The listener under Linux is configured differently, but its functionality is the same. There is no concept of a network name resource in Pacemaker, nor is an object created in AD DS; there is just an IP address resource created in Pacemaker that can run on any of the nodes. An entry associated with the IP resource for the listener in DNS with a “friendly name” needs to be created. The IP resource for the listener will only be active on the server hosting the primary replica for that availability group.
If Pacemaker is used and an IP address resource is created that is associated with the listener, there will be a brief outage as the IP address stops on the one server and starts on the other, whether it is automatic or manual failover. While this provides abstraction through the combination of a single name and IP address, it does not mask the outage. An application must be able to handle the disconnect by having some sort of functionality to detect this and reconnect.
However, the combination of the DNS name and IP address is still not enough to provide all the functionality that a listener on a WSFC provides, such as read-only routing for secondary replicas. When configuring an AG, a “listener” still needs to be configured in SQL Server. This can be seen in the wizard as well as the Transact-SQL syntax. There are two ways that this can be configured to function the same as on Windows:
- For an AG with a cluster type of External, the IP address associated with the “listener” created in SQL Server should be the IP address of the resource created in Pacemaker.
- For an AG created with a cluster type of None, use the IP address associated with the primary replica.
The instance associated with the provided IP address then becomes the coordinator for things like the read-only routing requests from applications.
Interoperability with Windows-based availability groups and replicas
An AG that has a cluster type of External or one that is WSFC cannot have its replicas cross platforms. This is true whether the AG is SQL Server Standard or SQL Server Enterprise. That means in a traditional AG configuration with an underlying cluster, one replica cannot be on a WSFC and the other on Linux with Pacemaker.
An AG with a cluster type of NONE can have its replicas cross OS boundaries, so there could be both Linux- and Windows-based replicas in the same AG. An example is shown here where the primary replica is Windows-based, while the secondary is on one of the Linux distributions.
A distributed AG can also cross OS boundaries. The underlying AGs are bound by the rules for how they are configured, such as one configured with External being Linux-only, but the AG that it is joined to could be configured using a WSFC. Consider the following example: