LATCH_SH

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Description:

This wait type occurs when a thread is waiting for access to a non-page data structure so that it can read the data structure. The thread cannot get access to the data structure because another thread has it latched in exclusive mode. For a full description of non-page latches and investigating them, see the LATCH_EX wait type.

(Books Online description: “Occurs when waiting for a SH (share) latch. This does not include buffer latches or transaction mark latches. A listing of LATCH_* waits is available in sys.dm_os_latch_stats. Note that sys.dm_os_latch_stats groups LATCH_NL, LATCH_SH, LATCH_UP, LATCH_EX, and LATCH_DT waits together.”)

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Added in SQL Server version:

Pre-2005/2005

Removed in SQL Server version:

N/A

Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 34 in all versions through 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:

For a full description of non-page latches and investigating them, see the LATCH_EX wait type.

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

1. Waiting for the FGCB_ADD_REMOVE latch to determine which filegroup file to allocate from (in this case, while copying a LOB value)
2. Waiting for the TRACE_CONTROLLER latch to read the list of active Profiler traces (in this case, while periodically flushing them)

And many, many more stacks around the set of non-page latches.

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

1. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   LatchBase::Suspend+633
   LatchBase::AcquireInternal+415
   FGCB::GetAllocFCB+7a
   AllocationReq::AllocatePages+175c
   AllocationReq::Allocate+f3
   CompactSpaceAllocator::AllocateAndFixPage+338
   CompactSpaceAllocator::AllocateAndFixPageInternal+ae
   CompactSpaceAllocator::FindSpaceAndFixPage+134
   BlobBase::AllocDataNode+26f
   Blob::InsertNonFull+34a
   Blob::InsertWholePage+177
   Blob::Insert+236
   BlobManager::ReplaceAtWithILB+5d0
   LockBytesSS::ReplaceAtWithILB+17
   CMainIlb::ReplaceAtWithILB+90
   CBlobHandleFactories::CopyIlbRaiseError+70
   CBlobHandleHelper::CopyBlobHandleIntoTempOrInlined+1f2
   CBlobHandleManagerCommon::PxvarPreprocessInternal+126
   CValRowNoHrow::SetDataX+28
2. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   LatchBase::Suspend+633
   LatchBase::AcquireInternal+415
   CTraceController::IncrementalFlush+7b
   CTraceManagementTask::Invoke+83
   CTraceBatchTask::Task+259
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+a8/li>