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