PAGEIOLATCH_SH

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Description:

This wait type is when a thread is waiting for the read of a data file page from disk to complete, and the thread is going to read (not modify) the page structure once it is in memory (SH = SHare mode). The Latches Whitepaper in the sidebar on the right has a description of all latch modes and their compatibility with other latch modes.

(Books Online description: “Occurs when a task is waiting on a latch for a buffer that is in an I/O request. The latch request is in Shared mode. Long waits may indicate problems with the disk subsystem.”)

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:

The map_key value in sys.dm_xe_map_values is 66 in 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:

The knee-jerk reaction to this wait type is that it must be the I/O subsystem that has a problem, and even the Books Online description mentions this. It’s true that long waits for I/Os indicate the I/O subsystem is overloaded, but it’s more likely that the problem is in SQL Server than with the I/O subsystem. You need to think about *why* SQL Server is doing so many reads.

I wrote a long article on troubleshooting PAGEIOLATCH_XX waits on sqlperformance.com – basically identifying the query that’s doing the reads and then figuring out which table is being read from and why that data isn’t already in memory. You can get the workflow for identifying a table from a page resource (from sys.dm_os_waiting_tasks) in this blog post.

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

1. Reading a heap data page (in this case, during a scan)
2. Reading an index leaf-level page (in this case, during a scan)
3. Reading an IAM page (in this case, to get the next extent as part of an unordered scan of an index)
4. Reading a text page (in this case, as part of retrieving table statistics for the Query Optimizer during query compilation)

And many, many more similar stacks to do with reading data file pages from disk.

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

1. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   LatchBase::Suspend+633
   LatchBase::AcquireInternal+415
   BUF::AcquireLatch+78
   BPool::Get+ea
   FixPageNotThruLinkage+1b3
   HeapPageManager::GetNextPage+1ea
   HeapDataSetSession::GetNextRowValuesInternal+52f
   RowsetNewSS::FetchNextRow+197
   CQScanRowsetNew::GetRowWithPrefetch+51
   CQScanTableScanNew::GetRow+8a
   CQueryScan::GetRow+81
   CXStmtQuery::ErsqExecuteQuery+36a
   CXStmtSelect::XretExecute+2f7
   CMsqlExecContext::ExecuteStmts<0,1>+864
   CMsqlExecContext::FExecute+a48
   CSQLSource::Execute+866
2. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   LatchBase::Suspend+633
   LatchBase::AcquireInternal+415
   BUF::AcquireLatch+78
   BPool::Get+ea
   IndexPageManager::GetNextPage+438
   IndexRowScanner::MoveKeyOrderToRowOnNextPage+16c
   IndexRowScanner::MoveToRowOnNextPage+23b
   IndexDataSetSession::GetNextRowValuesInternal+105b
   DatasetSession::GetNextRowValuesNoLock+ce
   RowsetNewSS::FetchNextRow+18c
   CQScanRowsetNew::GetRowWithPrefetch+51
   CQScanRangeNew::GetRow+15b
   CQScanStreamAggregateNew::GetRowHelper+1fc
   CQScanStreamAggregateNew::GetCalculatedRow+21
   CQueryScan::StartupQuery+240
   CXStmtQuery::SetupQueryScanAndExpression+2bd
   CXStmtQuery::InitForExecute+34
   CXStmtQuery::ErsqExecuteQuery+36a
   CXStmtSelect::XretExecute+2f7
   CMsqlExecContext::ExecuteStmts<1,1>+400
3. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   LatchBase::Suspend+633
   LatchBase::AcquireInternal+415
   BUF::AcquireLatch+78
   BPool::Get+ea
   IAMPageRef::Fix+b5
   AllocScan::GetNextScanIAM+b9
   AllocScan::GetNextScanExtent+48b
   AllocScan::MoveToStartOfScan+1ca
   UnOrderPageIdScanner::GetNextPageId+31f
   UnOrderPageScanner::SetupSubScanner+114
   UnOrderPageScanner::StartScan+10a
   IndexPageManager::CreateScanner+257
   IndexRowScanner::SetRangeAllocationOrdered+113
   IndexDataSetSession::StartScan+413
   RowsetNewSS::EnsureStartOfScanLong+97
   RowsetNewSS::FetchNextRow+36
   CQScanTableScanNew::GetRow+f2
   CQScanSortNew::BuildSortTable+87
   CQScanSortNew::OpenHelper+c0
   CQScanNLJoinNew::Open+24
4. SOS_Task::PostWait+90
   EventInternal::Wait+1f9
   LatchBase::Suspend+633
   LatchBase::AcquireInternal+415
   BUF::AcquireLatch+78
   BPool::Get+ea
   BlobBase::FixNode+210
   Blob::RWBlob+30e
   Blob::ReadWriteAt+139
   BlobManager::ReadAtInternal+1d2
   LockBytesSS::ReadAt+40
   CMEDScanLob::CbGetNext+52
   CStatMan::Init+116
   CTableMetadata::LoadStats+159
   CTableMetadata::LoadStatisticsBlob+17f
   CTableMetadata::GatherStatisticsBlobs+2b5
   CTableMetadata::LoadMetadata+398
   OptimizerUtil::LoadNewStats+543