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PREEMPTIVE_OS_FILEOPS

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Description:

This wait type is a generic wait for when a thread is calling one of several Windows functions related to the file system.

(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 442 in 2008 and 2008 R2, 490 in 2012, and 506 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. Retrieving volume info (in this case, while populating the in-memory FCB (File Control Block) while starting up databases during instance startup)
  2. Retrieving the volume sector size (in this case, while starting up databases during instance startup)
  3. Retrieving volume info (in this case, while initializing a BACKUP operation)

And many other similar call stacks.

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

  1. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    GetVolumeInfoForFile+33c
    FCB::Init+11d
    FileMgr::UpdateFCBMap+22f
    FileMgr::BuildFCBMapFromSysaltfiles+39b
    FileMgr::StartupPreRecovery+554
    RecoveryUnit::PreRecovery+11c3
    StartupCoordinator::NotifyPhaseStart+3a
    DBTABLE::Startup+1045
    DBMgr::StartupDB+6c6
    DBMgr::StartDBsInParallel+511
    DBMgr::ParallelDbStartupEntryPoint+10f
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+ab
  2. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    FCB::GetActualSectorSize+5bc
    FCB::GetActualSectorSize+58
    FCB::AsyncRead+574
    FCB::PageRead+bc
    FCB::RefreshHeaderFields+2ce
    FCB::Startup+d4
    FCB::StartPrimaryFile+109
    DBTABLE::Startup+51b
    DBMgr::StartupDB+6c6
    DBMgr::StartDBsInParallel+511
    DBMgr::ParallelDbStartupEntryPoint+10f
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+ab
  3. SOS_Task::PopWait+b1
    AutoSwitchPreemptive::~AutoSwitchPreemptive+95
    StorageUnitVolume::Init+25a
    BackupVolumeList::IncludeVolume+4c
    BackupFileDesc::AssociateWithVolume+57
    BackupOperation::LoadFileList+13e
    BackupOperation::SnapshotMetadata+275
    BackupDatabaseOperation::PerformBackupInitiationSteps+1d
    BackupEntry::BackupDatabase+304
    CStmtDumpDb::XretExecute+ef
    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