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Use the system_health session

  • Článek

Applies to: SQL Server Azure SQL Managed Instance

The system_health session is an Extended Events session that is included by default with SQL Server and Azure SQL Managed Instance. This session starts automatically when the Database Engine starts, and runs without any noticeable performance overhead. The session collects system data that you can use to help troubleshoot performance issues in the Database Engine.

Important

We recommend that you don't stop, alter, or delete the system_health session. Any changes made to the system_health session settings might be overwritten by a future product update.

The session collects information that includes the following information:

  • The sql_text and session_id for any sessions that encounter an error that has a severity >= 20.
  • The sql_text and session_id for any sessions that encounter a memory-related error. The errors include 17803, 701, 802, 8645, 8651, 8657 and 8902.
  • A record of any non-yielding scheduler problems. These appear in the SQL Server error log as error 17883.
  • Any deadlocks that are detected, including the deadlock graph.
  • The callstack, sql_text, and session_id for any sessions that have waited on latches (or other interesting resources) for > 15 seconds.
  • The callstack, sql_text, and session_id for any sessions that have waited on locks for > 30 seconds.
  • The callstack, sql_text, and session_id for any sessions that have waited for a long time for preemptive waits. The duration varies by wait type. A preemptive wait is where SQL Server is waiting for external API calls.
  • The callstack and session_id for CLR allocation and virtual allocation failures.
  • The ring buffer events for the memory broker, scheduler monitor, memory node OOM, security, and connectivity.
  • System component results from sp_server_diagnostics.
  • Instance health collected by scheduler_monitor_system_health_ring_buffer_recorded.
  • CLR Allocation failures.
  • Connectivity errors using connectivity_ring_buffer_recorded.
  • Security errors using security_error_ring_buffer_recorded.

Note

For more information on deadlocks, see the Deadlocks guide. For more information on SQL error messages, see Database Engine events and errors.

View the system_health session data

The session uses both the ring buffer target and event file target to store the data. The event file target is configured with a maximum size of 5 MB and a file retention policy of 4 files.

To view the session data from the ring buffer target with the Extended Events user interface available in SQL Server Management Studio, see Advanced Viewing of Target Data from Extended Events in SQL Server - Watch live data.

To view the session data from the ring buffer target with Transact-SQL, use the following query:

SELECT CAST(xet.target_data as xml) AS target_data
FROM sys.dm_xe_session_targets xet
JOIN sys.dm_xe_sessions xe
ON xe.address = xet.event_session_address
WHERE xe.name = 'system_health'

To view the session data from the event file, use the Extended Events event viewer UI available in SQL Server Management Studio. For more information, see View event data in SQL Server Management Studio.

Restore the system_health session

If you delete the system_health session, you can restore it by executing the u_tables.sql script. This file is located in the following folder, where C: represents the drive where you installed the SQL Server program files, and MSSQLnn the major version of SQL Server:

C:\Program Files\Microsoft SQL Server\MSSQLnn.\<instanceid>\MSSQL\Install

After you restore the session, you must start it by using the ALTER EVENT SESSION statement or by using the Extended Events node in Object Explorer. Otherwise, the session starts automatically the next time that you restart the SQL Server service.

The system_health session in Azure SQL

In Azure SQL Managed Instance, the system_health session data can be viewed by right-clicking either event_file or ring_buffer target in Object Explorer, and selecting View Target Data.

There's no built-in system_health Extended Event session in Azure SQL Database, but you can use the sys.fn_xe_file_target_read_file() function to read from Extended Event sessions you create yourself and store in Azure Storage. For a walkthrough, see Event File target code for Extended Events in Azure SQL Database.

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