LCK_M_SCH_M

Overview

Occurs when waiting for a schema modification (Sch-M) lock. Blocks nearly everything. Typically from DDL operations (ALTER TABLE, CREATE INDEX) running concurrently with active queries.

Key Metrics

Metric	Description
Wait Type	LCK_M_SCH_M
Category	Blocking
Severity	HIGH

Troubleshooting

1. Identify the source

Use the following query to identify the top queries contributing to LCK_M_SCH_M waits:

SELECT TOP 10
 [Wait Type] = wait_type,
 [Wait Seconds] = wait_time_ms / 1000,
 [Wait Count] = waiting_tasks_count,
 [Avg Wait Ms] = wait_time_ms / NULLIF(waiting_tasks_count, 0)
FROM sys.dm_os_wait_stats
WHERE wait_type = 'LCK_M_SCH_M'
ORDER BY wait_time_ms DESC;

2. Related diagnostics

• Review overall wait statistics using sys.dm_os_wait_stats
• Check session-level waits via sys.dm_exec_session_wait_stats
• Correlate with sys.dm_exec_requests for blocking analysis

See Also

• sys.dm_os_wait_stats (Transact-SQL)
• sys.dm_exec_session_wait_stats (Transact-SQL)
• sys.dm_os_waiting_tasks (Transact-SQL)