PARALLEL_REDO_DRAIN_WORKER

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Description:

This wait type is when the main parallel redo thread for a database (on a standalone instance or Availability Group replica) needs to wait for all outstanding log record redo operations to be completed by other parallel redo threads. This can happen when redoing certain logged operations like a checkpoint of log file size change and is part of normal parallel redo operations.

(Books Online description: “Internal use only.”)

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Added in SQL Server version:

2016

Removed in SQL Server version:

N/A

Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 1132 in 2016 RTM. After 2016 RTM, you must check the DMV to get the latest value as some map_key values have changed in later builds.

Other information:

This wait type is one that I usually filter out as a benign wait when doing wait statistics analysis as it is documented as not being indicative of a performance issue.

You can read more about parallel redo in this blog post.

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

1. 1-2: call stacks during parallel redo, in this case during crash recovery of a database on a standalone instance

And various other similar call stacks.

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

1. SOS_Task::PostWait+0x6f
   WaitableBase::Wait+0x178
   ParallelRedoManager::DrainRedoWorkers+0x119
   RecoveryMgr::RedoPass+0x18f5
   RecoveryMgr::PhysicalRedo+0x8d0
   RecoveryUnit::PhysicalRecovery+0x38f
   StartupCoordinator::NotifyPhaseStart+0x4a
   DBTABLE::Startup+0x138b
   DBMgr::StartupDB+0x7c9
   DBMgr::StartDBsInParallel+0x5c0
   DBMgr::ParallelDbStartupEntryPoint+0xf7
   SOS_Task::Param::Execute+0x232
   SOS_Scheduler::RunTask+0xa5
2. SOS_Task::PostWait+0x6f
   WaitableBase::Wait+0x178
   ParallelRedoManager::DrainRedoWorkers+0x119
   RecoveryMgr::RedoLogicalOp+0x94d
   RecoveryMgr::RedoLogRecord+0x488
   RecoveryMgr::RedoPass+0x18c5
   RecoveryMgr::PhysicalRedo+0x8d0
   RecoveryUnit::PhysicalRecovery+0x38f
   StartupCoordinator::NotifyPhaseStart+0x4a
   DBTABLE::Startup+0x138b
   DBMgr::StartupDB+0x7c9
   DBMgr::StartDBsInParallel+0x5c0
   DBMgr::ParallelDbStartupEntryPoint+0xf7
   SOS_Task::Param::Execute+0x232
   SOS_Scheduler::RunTask+0xa5