Failover clustering and AlwaysOn Availability

In an SQL Server cluster environment, two different kinds of clustering are supported; AlwaysOn Failover Cluster Instances (FCI) and AlwaysOn Availability Groups (AAGs). An SQL AlwaysOn failover cluster instance provides high availability and disaster recovery at the SQL Server level. AlwaysOn Availability Groups (AAG) provide high availability and disaster recovery at SQL database level. Data Protection for SQL Server protects availability databases in both AlwaysOn failover cluster instances and in an AAG.

An AlwaysOn node manages backups of availability databases. This node is a shared node that allows data backups and restores of availability databases from any database replica in the cluster. Data Protection for SQL Server treats a backup as originating on a single SQL Server regardless of which node of the cluster is backed up.

Failover cluster instances

In a Windows failover cluster instance with multiple SQL Server instances, the storage is shared and can be accessed by all systems in the cluster. However, only one server in the cluster at a time runs SQL Server services. When you run a backup, the backup runs on the same server in the cluster that is running the SQL Server service. Therefore, when you run a backup, for example by using the Windows scheduler, it must run on this same server in the cluster where the SQL Server instance is active.

AlwaysOn availability groups (AAGs)

An AAG can contain a set of primary databases and multiple copies of the set of primary databases, called secondary databases. You can have as many as nine online copies of a database (one primary and up to eight secondary replicas) in an AAG. Databases in an availability group are called availability databases, and they fail over together as a group. Unlike a failover cluster, in an AAG, storage is not shared because SQL Server uses log shipping to replicate data from the primary database to the secondary database instances.

You can use AAGs with SQL AlwaysOn failover cluster instances to complete the following tasks:

In an AAG, you can deploy a group of single or clustered server instances, each holding a copy of all databases
You can use synchronous and asynchronous replication
You can use log shipping. When a transaction occurs on the primary database, it is shipped to the secondary databases.
You can use automatic and manual failover modes

Cluster setup considerations for AAGs

To set up AAGs in a Microsoft Windows failover cluster environment or in a Veritas cluster server cluster environment, follow these guidelines:

Install Data Protection for SQL Server on each cluster node and configure each node identically. Specify identical configurations in the Data Protection for SQL Server options file.
Ensure that each availability replica of an availability group is on a different node in the same Windows failover cluster environment.
Use the Configuration Wizard to register an AlwaysOn node on the IBM Spectrum Protect™ server. To do so manually, issue the register node command on the IBM Spectrum Protect server.
To access a clustered SQL Server, identify the virtual server name and specify that name in Data Protection for SQL Server.
If you use the IBM Spectrum Protect scheduler to automate data backups, install the scheduler service on each node of the cluster to enable failover support.
You cannot restore a VSS backup to an alternative instance. Restore VSS backups on the same SQL Server instance where the snapshot is taken.
Tip: VSS and legacy full backups of availability databases on secondary replicas are copy-only. The copy-only option is not automatically used with log backups because you can run log backups that truncate logs on secondary replicas.