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This wait type is when the main database mirroring thread is waiting for something to do. This wait only occurs when database mirroring is configured, and if nothing is happening on the principal server, then individual waits could be very long.
(Books Online description: “Occurs when database mirroring waits for events to process.”)
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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 140 in 2008 and 2008 R2, and 144 in 2012 and 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.
I have not seen this wait type be a noticeable contention point.
This wait type is one that I usually filter out as a benign wait when doing wait statistics analysis.
Known occurrences in SQL Server (list number matches call stack list):
- Waiting for a database mirroring event
Abbreviated call stacks (list number matches known occurrences list):