What’s the issue?
The AlwaysOn_health Extended Events (XE) session is a built-in SQL Server diagnostic session that is automatically created on any instance hosting an Always On Availability Group. It captures critical events related to availability group health, including replica state changes, failover validations, lease expirations, connectivity errors, and redo blocking.
When this session is in a stopped state, SQL Server is no longer recording these diagnostic events to its rollover trace files. This can occur if the session was manually stopped, if it was created without the STARTUP_STATE=ON property, or if it failed to start after a service restart or manual intervention.
Why is this a problem?
Without the AlwaysOn_health session running, you lose the primary built-in source of historical diagnostic data for troubleshooting Availability Group issues. If a failover, synchronization problem, connectivity disruption, or lease timeout occurs, you will have no retrospective record of what happened, making root-cause analysis extremely difficult or impossible.
The Availability Group Dashboard in SQL Server Management Studio also relies on this session’s data to display health information, so stopping it degrades your monitoring visibility. In environments where high availability is critical, this is effectively flying blind during the exact moments when diagnostics matter most.
What should you do about this?
Start the session immediately using ALTER EVENT SESSION [AlwaysOn_health] ON SERVER STATE = START; and verify it’s running by querying sys.dm_xe_sessions. To prevent it from remaining stopped after future service restarts, ensure the session is configured to auto-start by setting STARTUP_STATE = ON in its properties.