LCK_M_SCH_M_ABORT_BLOCKERS

(Republishing, or using this info in a commercial product/website, is prohibited without permission. All other uses are permitted. If in doubt, please ask.)

(Back to main page…)

Description:

This wait type is when a thread is running an online index/partition switch operation and is blocked by other operations while waiting for the final Schema Modification (also called Schema Modify) lock. In this case, the WAIT_AT_LOW_PRIORITY option has been specified, the MAX_DURATION time has expired, and the blocking operations are being rolled back.

You can read more about the WAIT_AT_LOW_PRIORITY option here.

(Books Online description: “Occurs when a task is waiting to acquire a Schema Modify lock with Abort Blockers. (Related to the low priority wait option of ALTER TABLE and ALTER INDEX.)”)

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:

2014

Removed in SQL Server version:

N/A

Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 955 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 type be a noticeable contention point.

The duration of each wait of this type will be how long it takes for all operations that are blocking the online index/partition switch operation to roll back (following immediately from an LCK_M_SCH_M_LOW_PRIORITY wait from while the online index/partition switch operation was waiting at low priority).

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

1. Waiting for the Schema Modification lock during an online index operation (same stack as for LCK_M_SCH_M_LOW_PRIORITY except for a different location in the MDL::LockObjectLocal function)
2. Waiting for the Schema Modification lock during a partition switch operation and the blocking operations are rolling back (same stack as for LCK_M_SCH_M_LOW_PRIORITY except for a different location in the MDL::LockObjectLocal function)

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

1. EventInternal<SuspendQueueSLock>::Wait+0x1fb
   LockOwner::Sleep+0x485
   lck_lockInternal+0x10ed
   MDL::LockObjectLocal+0xdbe
   CMEDCacheEntryFactory::AcquireLock+0x61
   CMEDCatalogObject::GetCachedObjectById+0xe2
   CMEDProxyDatabase::GetObjectByObjectId+0x91
   CIndexDDL::CleanupIndices+0xcb
   CIndexDDL::Rebuild+0xa25
   RegularIndexRebuildHelper+0x35e
   CISRegularIndex::Rebuild+0x8f
   CStmtAlterIndex::XretExecute+0x16e3
   CMsqlExecContext::ExecuteStmts<1,1>+0x427
   CMsqlExecContext::FExecute+0xa43
   CSQLSource::Execute+0x86c
   process_request+0xa57
   process_commands+0x4a3
   SOS_Task::Param::Execute+0x21e
   SOS_Scheduler::RunTask+0xab
2. SOS_Task::PostWait+0x9e
   EventInternal<SuspendQueueSLock>::Wait+0x1fb
   LockOwner::Sleep+0x485
   lck_lockInternal+0x10ed
   MDL::LockObjectLocal+0xdbe
   CMEDCacheEntryFactory::AcquireLock+0x61
   CMEDCatalogObject::GetCachedObjectById+0xe2
   CMEDProxyDatabase::GetObjectByObjectId+0x91
   CStmtAlterTable::XretExecute+0xb89
   CMsqlExecContext::ExecuteStmts<1,1>+0x427
   CMsqlExecContext::FExecute+0xa43
   CSQLSource::Execute+0x86c
   process_request+0xa57
   process_commands+0x4a3
   SOS_Task::Param::Execute+0x21e
   SOS_Scheduler::RunTask+0xab