SQL Server 2012 - Always On - a glance

AlwaysOn = HA + DR 

Features:

• Multi-Database Failovers .
• Multiple Secondaries.
• Active Secondaries .
• Integrated HA Management.
• Does not Require any Shared Storage . 

Benefits:

• Supports alternative availability modes (Sync / Async)
• Supports several forms of availability group fail-over (auto/manual/forced)
• Readable secondary replicas (Yes/No/Read Intent)
• Supports automatic page repair for protection against page corruption. Supports encryption and compression, which provide a secure, high performing transport
• Backups from secondary replicas

Performance Improvements:

• Data Latency (secondary replica)
• Read-Only Workload Impact (secondary replica)
• Statistics for Read-Only Access Databases (secondary replica)

Limitations and Restrictions:

Availability group Restrictions :

• Availability replicas must be hosted by different nodes of one WSFC cluster.
• Unique availability group name
• Maximum number of availability groups and availability databases per computer(as per white papers 10 Availability Group with 10 DB’s each )

Cross DB connection:

• Cross DB access can be given as a work around cannot be given directly between availability groups.
• For SQL authentication the cross DB can be performed through rev- logins only.
• Cross availability group DB connection will not work if any of the referenced AG fails.

Availability Database Restrictions :

• Impact on add-file operations.
• RESTORE WITH MOVE

Network restrictions :

• An Availability group to support automatic fail-over, the secondary replica that is the automatic-failover partner must be in the SYNCHRONIZED state.
• If the network link to this secondary replica fails (even intermittently), the replica enters the UNSYNCHRONIZED state and cannot begin to re-synchronize until the link is restored.
• If the WSFC cluster requests an automatic fail-over while the secondary replica is unsynchronized, automatic fail-over will not occur.