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FCB_REPLICA_READ

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Description:

This wait type is when a thread is waiting for a read request through the context of a database snapshot.

(Books Online description: “Occurs when the reads of a snapshot (or a temporary snapshot created by DBCC) sparse file are synchronized.”)

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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 176 in 2008 and 2008 R2, and 180 in 2012 and 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.

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

  1. Reading from a database snapshot (in this case, during the first phase of crash recovery that pre-reads affected pages into the buffer pool, as part of the creation of a the hidden database snapshot that DBCC CHECKDB creates)
  2. Reading from a database snapshot (in this case, while running a DBCC CHECKDB which uses a hidden database snapshot)

And other similar call stacks.

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

  1. SOS_Task::PostWait+9e
    EventInternal<SuspendQueueSLock>::Wait+1fb
    FCBReplicaSync::StartRead+19e
    FCB::AsyncRead+1b0
    RecoveryUnit::ScatterRead+89
    BPool::GetFromDisk+719
    BPool::ReadAhead+7e
    ForwardLogDataReadAhead::DoLogDataReadAhead+127
    LogConsumer::ProcessLogBlocks+387
    LogConsumer::ReadAhead+78c
    LogConsumer::RefCurrentLogBlock+1fb
    LogConsumer::GetNextLogBlock+ae4
    SQLServerLogIterForward::GetNext+208
    SQLServerCOWLogIterForward::GetNext+74
    RecoveryMgr::RedoPass+648
    RecoveryMgr::PhysicalRedo+5ec
    RecoveryUnit::PhysicalRecovery+341
    StartupCoordinator::NotifyPhaseStart+3a
    DBTABLE::ReplicaCreateStartup+26d
    DBMgr::SyncAndLinkReplicaRecoveryPhase+787
    DBMgr::CreatePhasedTransientReplica+4e1
    DBMgr::CreateTransientReplica+116
  2. SOS_Task::PostWait+90
    EventInternal<SuspendQueueSLock>::Wait+1f9
    FCBReplicaSync::StartRead+19e
    FCB::ScatterRead+14e
    RecoveryUnit::ScatterRead+e2
    BPool::GetFromDisk+799
    MultiObjectScanner::GetNextPageAndReadAhead+3bb
    MultiObjectScanner::GetNext+98
    MultiObjectScanner::GetNextPageAndBatch+26f
    CheckTables::ProcessNextData+1ad
    CheckAggregateSingleInstance::GetNextFact+247
    CTRowsetInstance::FGetNextRow+3c
    CUtRowset::GetNextRows+a0
    CQScanRmtScanNew::GetRowHelper+3b5
    CQScanXProducerNew::GetRowHelper+53
    CQScanXProducerNew::GetRow+15
    FnProducerOpen+57
    FnProducerThread+851
    SubprocEntrypoint+a59
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+a8