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Just as the description below states. I/O for a database must be paused (using the undocumented DBCC FREEZE_IO command) by an external process that is backing up the database files directly, instead of using the VDI interface to perform a ‘SQL Server’ backup.

(Books Online description: “Occurs when a task is waiting for all outstanding I/O to complete, so that I/O to a file can be frozen for snapshot backup.”)

Questions/comments on this wait type? Click here to send Paul an email, especially if you have any information to add to this topic.

Added in SQL Server version:


Removed in SQL Server version:


Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 236 in 2008 and 2008 R2, 243 in 2012, and 250 in 2014 RTM. After 2014 RTM, you must check the DMV to get the latest value as some map_key values have changed in later builds.

Other information:

There will be one wait per second, lasting for a second, for each second that the thread has to wait for all outstanding I/Os to complete.

Known occurrences in SQL Server (list number matches call stack list):

  1. Waiting for all outstanding I/Os to complete (in this case, while executing the undocumented DBCC FREEZE_IO command)

Abbreviated call stacks (list number matches known occurrences list):

  1. SOS_Task::PostWait+9e