ASYNC_IO_COMPLETION

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Description:

This wait type represents a general purpose asynchronous I/O operation that is not using the buffer pool, or for checkpoint operations.

(Books Online description: “Occurs when a task is waiting for I/Os to finish.”)

Questions/comments on this wait type? Click here to send Paul an email, especially if you have any information to add to this topic.

Added in SQL Server version:

Pre-2005/2005

Removed in SQL Server version:

N/A

Extended Events wait_type value:

98 (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:

As general guidance for I/O wait types, if the wait time is higher than acceptable I/O latencies for your environment, investigate whether the I/O subsystem is overloaded or has a configuration problem. See the PAGEIOLATCH_SH wait type for more information.

For stacks 1-3 below, there is one of each of these waits per data backup, and for the third stack, this wait will be almost the duration of the entire data backup operation (which can skew the average wait time for this wait type). See here for more details.

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

1. The checkpoint at the start of a data backup
2. Reading the GAM pages from the data files to figure out what extents to back up during a full backup
3. Reading the actual data from the data files during a data backup (the writes to the backup files are tracked by BACKUPIO waits)
4. Reading log records as part of replaying log on a mirror database
5. The checkpoint at the start of creating the transient database snapshot when a DBCC CHECK* command runs
6. Creating database files during database creation (one wait covers all file creation)
7. A regular checkpoint

And many more similar call stacks.

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

1. SOS_Task::PostWait+90
   EventInternal<SuspendQueueSLock>::Wait+1f9
   AsynchronousDiskPool::WaitUntilDoneOrTimeout+fb
   CheckpointDB2+2cd
   BackupDatabaseOperation::PerformDataCopySteps+c2
   BackupEntry::BackupDatabase+2ca
   CStmtDumpDb::XretExecute+ef
   CMsqlExecContext::ExecuteStmts<1,1>+400
   CMsqlExecContext::FExecute+a33
   CSQLSource::Execute+866
2. SOS_Task::PostWait+90
   EventInternal<SuspendQueueSLock>::Wait+1f9
   AsynchronousDiskPool::WaitUntilDoneOrTimeout+fb
   BackupOperation::GenerateExtentMaps+34c
   BackupDatabaseOperation::PerformDataCopySteps+179
   BackupEntry::BackupDatabase+2ca
   CStmtDumpDb::XretExecute+ef
   CMsqlExecContext::ExecuteStmts<1,1>+400
   CMsqlExecContext::FExecute+a33
   CSQLSource::Execute+866
3. SOS_Task::PostWait+90
   EventInternal<SuspendQueueSLock>::Wait+1f9
   AsynchronousDiskPool::WaitUntilDoneOrTimeout+fb
   BackupOperation::BackupData+272
   BackupEntry::BackupDatabase+2ca
   CStmtDumpDb::XretExecute+ef
   CMsqlExecContext::ExecuteStmts<1,1>+400
   CMsqlExecContext::FExecute+a33
   CSQLSource::Execute+866
4. SOS_Task::PostWait+9e
   EventInternal<SuspendQueueSLock>::Wait+1fb
   LsBuffer::WaitForIoCompletion+e8
   LsBuffer::Quiesce+48
   LsMgr::ProcessSegment+79
   LsMgr::ProcessLogStream+79e
   LsMgr::ProcessLogStreamResponse+6e
   LsWorkRequest::Execute+47
   LsWorker::ThreadRoutine+152
   LsWorker::ThreadRoutine+1c7
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+a8
5. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   AsynchronousDiskPool::WaitUntilDoneOrTimeout+fb
   CheckpointDB2+2cd
   DBMgr::SyncAndLinkReplicaSetupPhase+812
   DBMgr::CreatePhasedTransientReplica+3b5
   DBMgr::CreateTransientReplica+254
   DBDDLAgent::CreateReplica+184
   UtilDbccCreateReplica+7a
   UtilDbccCheckDatabase+919
   DbccCheckDB+239
   DbccCommand::Execute+153
   CStmtDbcc::XretExecute+80d
   CMsqlExecContext::ExecuteStmts<1,1>+400
   CMsqlExecContext::FExecute+a33
   CSQLSource::Execute+866
6. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   AsynchronousDiskPool::WaitUntilDoneOrTimeout+fb
   AsynchronousDiskPool::WaitUntilDone+c
   DBMgr::CreateAndFormatFiles+46d
   CStmtCreateDB::CreateLocalDatabaseFragment+682
   DBDDLAgent::CreateDatabase+f7
   CStmtCreateDB::XretExecute+8fc
   CMsqlExecContext::ExecuteStmts<1,1>+400
   CMsqlExecContext::FExecute+a33
   CSQLSource::Execute+866
   process_request+73c
   process_commands+51c
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+a8
7. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   AsynchronousDiskPool::WaitUntilDoneOrTimeout+fb
   CheckpointDB2+2cd
   ProcessCheckpointRequest+1e8
   CheckpointHelper+162
   CheckpointLoopWorker+da
   RegisterCheckPtWorker+133
   CheckpointLoop+d3
   CheckpointThread+55
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+a8