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FT_MASTER_MERGE

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Description:

This wait is when a thread that is participating in a full-text master merge is waiting for other threads to complete. A master merge is when an ALTER FULLTEXT CATALOG … REORGANIZE is performed to merge smaller full-text indexes into one large index (done periodically to increase performance and reduce memory and disk space).

(Books Online description: “Full-text is waiting on master merge operation. Documented for informational purposes only. Not supported. Future compatibility is not guaranteed.”)

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Added in SQL Server version:

2008

Removed in SQL Server version:

N/A

Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 589 in 2008 and 2008 R2, 639 in 2012, and 655 in 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 be a noticeable contention point.

However, there is a known issue that can result in this wait type and failed master merges if many indexing and reorganize operations are run concurrently – see here for more details. The workaround is to stagger the indexing and reorganize operations.

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

  1. Waiting for other threads to complete during a master merge.

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

  1. SOS_Task::PostWait+6e
    EventInternal<SuspendQueueSLock>::Wait+23f
    CFTWIMasterMergeCoordinator::WaitForCompletion+61
    QueueMasterMergeAndWait+28d
    IFTSEndofCrawlMasterMerge+138
    CFTWICrawl::Close+a42
    CFTWICrawl::RemoveChildCrawl+187
    CFTWICrawl::Close+c9c
    CFTWIBatchComplete::ExecuteImpl+c3
    IFTSWorkerTask+214
    SystemTaskEntryPoint+331
    SOS_Scheduler::SystemTimerEntryPoint+bc
    SOS_Task::Param::Execute+292
    SOS_Scheduler::RunTask+a2