FCB_REPLICA_WRITE

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Description:

This wait type is when a thread is waiting for a synchronous write into a database snapshot by the buffer pool. This can occur in two ways:

1. Just before the page in the snapshot source database is going to be changed for the first time after the snapshot was created (the ‘push’ case in the Books Online description below)
2. When a page from the snapshot source database needs to be changed in the snapshot as part of running crash recovery into the snapshot when it is first created (the ‘pull’ case in the Books Online description below)

(Books Online description: “Occurs when the pushing or pulling of a page to a snapshot (or a temporary snapshot created by DBCC) sparse file is synchronized.”)

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 175 in 2008 and 2008 R2, and 179 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.

However, I can imagine a case where there are many database snapshots for a single source database, and lots of updates happening, that would cause a lot of these waits to happen and slow down the updates on the source database, as a pre-change page image must be synchronously pushed to all existing and relevant snapshots before the page can be changed in the source database. In that case, the solution would be to not have so many database snapshots on such a volatile source database.

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

1. Pushing a page into a snapshot (in this case, while inserting a record into an index page)
2. Pulling a page into a snapshot (in this case, as part of running crash recovery during the creation phase of a snapshot, where the snapshot is the hidden snapshot created by DBCC CHECKDB)
3. Pushing a page into a snapshot (in this case, while modifying bits in a differential bitmap during the initial checkpoint performed by a data backup)

And many other similar call stacks.

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

1. SOS_Task::PostWait+9e
   EventInternal<SuspendQueueSLock>::Wait+1fb
   FCBReplicaSync::StartWrite+18f
   FCB::CopyPageToReplicas+212
   BUF::CopyOnWrite+60
   BPool::PrepareToDirty+180
   PageRef::InsertRows+24f
   IndexPageRef::Insert+502
   InsertRecordOnPage+61b
   IndexDataSetSession::InsertSmallRecord+10ca
   IndexDataSetSession::InsertRowInternal+2de1
   DatasetSession::InsertRow+163
   RowsetNewSS::InsertRow+26
   CValRowMult::SetDataX+357
   sqlTsEs.dll!CEsExec::GeneralEval+1a9
   CQScanUpdateNew::GetRow+43d
   CQueryScan::GetRow+81
   CXStmtQuery::ErsqExecuteQuery+36d
   CXStmtDML::XretDMLExecute+31c
   CXStmtDML::XretExecute+ad
   CMsqlExecContext::ExecuteStmts<1,1>+427
   CMsqlExecContext::FExecute+a43
2. SOS_Task::PostWait+9e
   EventInternal<SuspendQueueSLock>::Wait+1fb
   FCBReplicaSync::StartWrite+18f
   FCB::PullPageToReplica+35
   FCB::CopyPageToReplicas+12c
   BUF::CopyOnWrite+60
   BPool::PrepareToDirty+180
   RecoveryMgr::RedoPageOperation+c44
   RecoveryMgr::RedoLogRecord+68b
   RecoveryMgr::RedoPass+7c6
   RecoveryMgr::PhysicalRedo+5ec
   RecoveryUnit::PhysicalRecovery+341
   StartupCoordinator::NotifyPhaseStart+3a
   DBTABLE::ReplicaCreateStartup+26d
   DBMgr::SyncAndLinkReplicaRecoveryPhase+787
   DBMgr::CreatePhasedTransientReplica+4e1
   DBMgr::CreateTransientReplica+116
   DBDDLAgent::CreateReplica+183
   UtilDbccCreateReplica+7a
   UtilDbccCheckDatabase+9a2
   DbccCheckDB+23f
   DbccCommand::Execute+153
3. SOS_Task::PostWait+9e
   EventInternal<SuspendQueueSLock>::Wait+1fb
   FCBReplicaSync::StartWrite+18f
   FCB::CopyPageToReplicas+212
   BUF::CopyOnWrite+60
   BPool::PrepareToDirty+180
   PageRef::ModifyBitsNonTransactional+1c2
   PageRef::SetDiffMapChangeBit+1ba
   PageRef::FlushDeltaCountsNonTransactionally+f6
   BTreeRow::UpdateCountsNonTransactional+14a
   FlushHoBtsDeltaCounts+d9c
   PageAndRowCountFlush::FlushAllocInfoNonTransactionally+559
   VisibleHoBt::FlushDeltaCountsNonTransactionally+fc
   VisibleHoBt::FlushDeltaCountsForCheckpoint+ec
   HoBtFactory::FlushDeltaCounts+176
   CheckpointRU2+31a
   AsynchronousDiskAction::DoFlushCache+98
   AsynchronousDiskAction::ExecuteDeferredAction+246
   AsynchronousDiskPool::PerformActionsSerially+61
   AsynchronousDiskPool::WaitUntilDoneOrTimeout+71
   CheckpointDB2+2eb
   BackupDatabaseOperation::PerformDataCopySteps+c2