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This wait type is when a thread is waiting for a read request through the context of a database snapshot.
(Books Online description: “Occurs when the reads of a snapshot (or a temporary snapshot created by DBCC) sparse file are synchronized.”)
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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 176 in 2008 and 2008 R2, and 180 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):
- Reading from a database snapshot (in this case, during the first phase of crash recovery that pre-reads affected pages into the buffer pool, as part of the creation of a the hidden database snapshot that DBCC CHECKDB creates)
- Reading from a database snapshot (in this case, while running a DBCC CHECKDB which uses a hidden database snapshot)
And other similar call stacks.
Abbreviated call stacks (list number matches known occurrences list):