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PREEMPTIVE_OS_FLUSHFILEBUFFERS

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Description:

This wait type is when a thread is calling the Windows FlushFileBuffers 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 451 in 2008 and 2008 R2, 499 in 2012, and 515 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. Flush all in-memory buffers for a file to disk (in this case, making sure all of a backup file is durably stored when finished a BACKUP operation)
  2. Flush all in-memory buffers for a file to disk (in this case, as part of running a DBCC WRITEPAGE command that bypasses the buffer pool)
  3. Flush all in-memory buffers for a file to disk (in this case, as part of shutting down a database that has the AUTO_CLOSE option set)
  4. Flush all in-memory buffers for a file to disk (in this case, as part of writing out a checkpoint file during a RESTORE operation)

And other similar call stacks.

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

  1. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    DBFlushFileBuffers+b9
    BackupDiskFile::RequestDurableMedia+d5
    BackupFile::Close+92
    BackupMediaIo::Close+1b7
    BackupStream::DoClose+48
    BackupStream::ThreadMainRoutine+153
    BackupThread::ThreadBase+51
    SubprocEntrypoint+a7f
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+ab
  2. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    FCB::FlushFileBuffersByHandle+190
    FCB::EnsureBuffersFlushedForClose+60
    FCB::Close+2c7
    DbccWritePage+d24
    DbccCommand::Execute+153
    CStmtDbcc::XretExecute+7cd
    CMsqlExecContext::ExecuteStmts<1,1>+427
    CMsqlExecContext::FExecute+a43
    CSQLSource::Execute+86c
    process_request+a57
    process_commands+4a3
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+ab
  3. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    FCB::FlushFileBuffersByHandle+190
    FCB::EnsureBuffersFlushedForClose+60
    FCB::Close+2c7
    FileMgr::Shutdown+52
    RecoveryUnit::Shutdown+283
    DBTABLE::Shutdown+2e9
    DBMgr::ShutdownDB+208
    DBMgr::CleanShutdownDB+51c
    TaskDatabaseAutoClose::ProcessTskPkt+386
    OnDemandTaskContext::ProcessTskPkt+3e2
    SystemTaskEntryPoint+426
    OnDemandTaskContext::FuncEntryPoint+25
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+ab
  4. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    FCB::FlushFileBuffersByHandle+190
    CheckpointableOperation::Save+23d
    BackupOperation::SaveCheckpoint+1e2
    BackupStream::ThreadMainRoutine+207
    BackupThread::ThreadBase+51
    SubprocEntrypoint+a7f
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+ab