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PREEMPTIVE_OS_WRITEFILE

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Description:

This wait type is when a thread is calling the Windows WriteFile function.

(Books Online description: N/A)

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Added in SQL Server version:

2008

Removed in SQL Server version:

N/A

Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 465 in 2008 and 2008 R2, 513 in 2012, and 529 in 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:

I have not seen this wait type be a noticeable contention point.

Note that when a thread calls out to Windows, the thread changes from non-preemptive (SQL Server controls the thread) to preemptive (Windows controls the thread) mode. The thread’s state will be listed as RUNNING, as SQL Server doesn’t know what Windows is doing with the thread.

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

  1. Writing synchronously to a SQL Trace file
  2. Writing synchronously to a database snapshot (as part of running a DBCC CHECKDB operation)

And other similar call stacks.

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

  1. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    CTraceIoBuf::SyncWritePreemptive+15f
    CTraceIoBuf::IncrementalFlush+1e2
    CTraceFileIoProvider::IncrementalFlush+70
    CTraceController::IncrementalFlush+14e
    CTraceManagementTask::Invoke+83
    CTraceBatchTask::Task+25c
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+ab
  2. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    FCB::SyncWritePreemptive+342
    FCB::PullPageToReplica+14c
    FCB::CopyPageToReplicas+12c
    BUF::CopyOnWrite+60
    BPool::PrepareToDirty+180
    BootPagePtr::GetAccess+6b9
    DBMgr::SyncAndLinkReplicaRecoveryPhase+440
    DBMgr::CreatePhasedTransientReplica+4e1
    DBMgr::CreateTransientReplica+116
    DBDDLAgent::CreateReplica+183
    UtilDbccCreateReplica+7a
    UtilDbccCheckDatabase+9a2
    DbccCheckDB+23f
    DbccCommand::Execute+153
    CStmtDbcc::XretExecute+7cd
    CMsqlExecContext::ExecuteStmts<1,1>+427
    CMsqlExecContext::FExecute+a43
    CSQLSource::Execute+86c
    process_request+a57
    process_commands+4a3