Comparison of AlWays On Failover Cluster Instances (AOFCI) and AlWays on Availability Groups(AOAG)

Comparison of Failover Cluster Instances and Availability Groups

Both are called AlWaysOn Features only ( FCI & AG),AlwaysOn is not a single, specific feature. Rather, it is a set of availability features whose most salient components are the Failover Cluster Instances feature and the Availability Groups feature. 

 Regardless of the number of nodes in the FCI, an entire FCI hosts a single replica within an availability group. The following table describes the distinctions in concepts between nodes in an FCI and replicas within an availability group.

	Nodes within an FCI	Replicas within an availability group
Uses WSFC cluster	Yes		Yes
Protection level	Instance		Database
Storage type	Shared		Non-shared1
Storage solutions	Direct attached, SAN, mount points, SMB		Depends on node type
Readable secondaries	No2		Yes
Applicable failover policy settings	WSFC quorum FCI-specific Availability group settings3		WSFC quorum Availability group settings
Failed-over resources	Server, instance, and database		Database only

1While the replicas in an availability group do not share storage, a replica that is hosted by an FCI uses a shared storage solution as required by that FCI. The storage solution is shared only by nodes within the FCI and not between replicas of the availability group.

2Whereas synchronous secondary replicas in an availability group are always running on their respective SQL Server instances, secondary nodes in an FCI actually have not started their respective SQL Server instances and are therefore not readable. In an FCI, a secondary node starts its SQL Server instance only when the resource group ownership is transferred to it during an FCI failover. However, on the active FCI node, when an FCI-hosted database belongs to an availability group, if the local availability replica is running as a readable secondary replica, the database is readable.

3Failover policy settings for the availability group apply to all replicas, whether it is hosted in a standalone instance or an FCI instance.

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