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BACKUP_LOG_REDO

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Description:

This latch class is when a thread in a multi-threaded restore operation is waiting for access to the control structure for redo of transaction log.

(Books Online description: “Used to synchronize backup log redo operations.”)

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Added in SQL Server version:

pre-2005/2005

Removed in SQL Server version:

N/A

Extended Events latch_class value:

Maps to the BACKUP_LOGREDO map_value in sys.dm_xe_map_values.

The map_key value in sys.dm_xe_map_values is 27 in 2008 and 2008 R2, and 22 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 latch be a noticeable contention point.

See the LATCH_EX wait type for more information about latches in general and how to troubleshoot them.

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

  1. Initiating redo for a log block from a stream in the backup
  2. Signifying the start of a VLF from a stream in the backup (a similar call stack will exist for the end of a VLF)

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

  1. XeSqlPkg::latch_suspend_end::Publish+138
    LatchBase::Suspend+16cb
    LatchBase::AcquireInternal+415
    BackupLogRedoMachine::SubmitLogBlockForRedo+6e
    RestoreSQLLogMachine::ConsumeData+172
    RestoreCopyMachine::CopyData+19f
    BackupMedium::RestoreLog+5fb
    BackupMedium::Restore+eb5
    BackupStream::DoRestore+6e
    BackupStream::ThreadMainRoutine+207
    BackupThread::ThreadBase+51
    SubprocEntrypoint+a59
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+a8
  2. XeSqlPkg::latch_suspend_end::Publish+138
    LatchBase::Suspend+16cb
    LatchBase::AcquireInternal+415
    BackupLogRedoMachine::RegisterLogFileBegin+5e
    BackupMedium::RestoreLog+5f2
    BackupMedium::Restore+eb5
    BackupStream::DoRestore+6e
    BackupStream::ThreadMainRoutine+207
    BackupThread::ThreadBase+51
    SubprocEntrypoint+a59
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+a8