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This wait type is when a thread is waiting for parallel threads that are replaying one or more log records on the mirror database in a database mirroring partnership. This wait only occurs on the mirror server.
(Books Online description: “Identified for informational purposes only. Not supported. Future compatibility is not guaranteed.”)
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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 137 in 2008 and 2008 R2, and 141 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.
Known occurrences in SQL Server (list number matches call stack list):
In all cases, waiting for parallel redo threads to complete an operation.
And many more similar call stacks.
Abbreviated call stacks (list number matches known occurrences list):