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LCK_M_SCH_S

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Description:

This wait type is when a thread is waiting to acquire a Schema Stability (also called Schema Share) lock on a resource and there is at least one other lock in an incompatible mode granted on the resource to a different thread.

General locking information:

(Books Online description: “Occurs when a task is waiting to acquire a Schema Share lock.”)

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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 1 in all versions through 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:

General guidance around troubleshooting lock waits:

  • It is not possible to determine the lock resource from the sys.dm_os_wait_stats output. You can see the resource from sys.dm_os_waiting_tasks (using my script) or looking at the resource_description field of sys.dm_tran_locks where the request_status is ‘WAIT’.
  • You can use the blocked process report to get more detailed information on queries that are waiting for locks for a specified threshold (see here).
  • Look to see what is at the head of the blocking chain (i.e. the thread that’s holding the lock that’s blocking everyone) using a script (plenty of them available online – I don’t have a preferred one). What is that thread waiting for? Fixing that wait may help unravel the blocking. For example, a thread may be holding locks and committing a transaction, but there’s a synchronous mirror with a slow I/O subsystem so the mirror log write takes a long time, making the transaction commit take longer, and the locks take longer to be released, causing blocking.
  • Look for lock escalation, where an UPDATE transaction has escalated to a table X lock, causing widespread blocking.
  • Look for index operations causing table locks, and consider using online index operations (or if already using them, consider the WAIT_AT_LOW_PRIORITY feature in 2014+).
  • Look for code that specifies a TABLOCK (causes a table Shared lock) or TABLOCKX (causes a table Exclusive lock) hint.
  • Look for application code that will cause locks to be acquired and then waits for user input, or fails to commit a transaction for a long time.
  • Consider creating nonclustered indexes to remove row locks from the underlying heap/clustered index.
  • Consider using snapshot isolation or read committed snapshot isolation to allow readers to not take S/IS locks and reduce blocking.
  • Check the correct isolation level is being used as REPEATABLE_READ and SERIALIZABLE will hold S/IS locks until the end of a transaction.
  • Check for accidental use of the SERIALIZABLE isolation level, from using distributed transactions or incorrectly scoped .Net TransactionScope objects.

Specific guidance for LCK_M_SCH_S waits:

  • For an Schema Stability lock, the resource will be a table.
  • The only lock that will block a Schema Stability lock is a SCH_M (Schema Modification) lock from an index build/rebuild or table/schema modification.

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

  1. Waiting for the Schema Stability lock (in this case, during an ALTER INDEX … REORGANIZE operation)
  2. Waiting for the Schema Stability lock (in this  case, during a partition switch operation)

And other similar call stacks.

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

  1. SOS_Task::PostWait+9e
    EventInternal<SuspendQueueSLock>::Wait+1fb
    LockOwner::Sleep+485
    lck_lockInternal+10ed
    GetHoBtLockInternal+5e0
    IndexDataSetSession::WakeUpInternal+2ac
    UtilDbccRebalanceSpaces+500
    UtilDbccReorgRowset+65d
    UtilDbccReorgIndex+608
    DDLAgent::ReorganizeIndex+cf
    CTableIndexUtil::ReorganizeIndex+2b4
    CStmtAlterIndex::XretExecute+15c2
    CMsqlExecContext::ExecuteStmts<1,1>+427
    CMsqlExecContext::FExecute+a43
    CSQLSource::Execute+86c
    process_request+a57
    process_commands+4a3
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+ab
  2. SOS_Task::PostWait+0x9e
    EventInternal<SuspendQueueSLock>::Wait+0x1fb
    LockOwner::Sleep+0x485
    lck_lockInternal+0x10ed
    MDL::LockObjectLocal+0xdbe
    CMEDCacheEntryFactory::AcquireLock+0x61
    CMEDCatalogObject::GetCachedObjectById+0xe2
    CMEDCatalogObject::GetCachedObjectByName+0x81
    CMEDProxySchema::GetObjectByName+0x82
    IMetadataAccess::GetMultiNameObjectHelper+0x714
    CMEDAccess::GetMultiNameObject+0xdf
    CTableAlter::CTableAlter+0x2e6
    CStmtAlterTable::CStmtAlterTable+0x78
    CCompPlan::FCompileStep+0xd19
    CSQLSource::FCompile+0x1487
    CSQLSource::FCompWrapper+0xdd
    CSQLSource::Transform+0x445
    CSQLSource::Execute+0x401
    process_request+0xa57
    process_commands+0x4a3
    SOS_Task::Param::Execute+0x21e
    SOS_Scheduler::RunTask+0xab