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This latch class is when a thread is waiting for access to the instance-wide list of transaction versioning information blocks (XVBs). Versioning is the Storage Engine mechanism that underpins snapshot isolation, online index operations, and a few other features.
(Books Online description: “Internal use only.”)
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Added in SQL Server version:
Removed in SQL Server version:
Extended Events latch_class value:
Maps to the XVB_LIST_SCAN map_value in sys.dm_xe_map_values.
The map_key value in sys.dm_xe_map_values is 124 in 2008 and 2008 R2, and 121 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 latch be a noticeable contention point.
See the LATCH_EX wait type for more information about latches in general and how to troubleshoot them.
Known occurrences in SQL Server (list number matches call stack list):
- Removing an XVB from the list once a transaction has committed and the XVB is no longer needed
And other similar call stacks.
Abbreviated call stacks (list number matches known occurrences list):