LCK_M_IX Wait Type

The LCK_M_IX wait type in SQL Server indicates that a session is waiting to acquire an Intent Exclusive (IX) lock on a resource. This type of lock is often encountered during operations that modify data, such as inserts, updates, or deletes.

The IX lock itself does not block other sessions from reading the data but signals that the session intends to modify specific rows or pages within a table or index. However, the LCK_M_IX wait means the requested IX lock is being blocked by another lock (often a shared (S) or exclusive (X) lock) held by a different session.

Understanding Locking in SQL Server

SQL Server uses various types of locks to ensure data integrity and concurrency control. Here’s a quick overview of locks related to LCK_M_IX :

  • Shared (S) Lock: Allows other sessions to read the data but not modify it.
  • Exclusive (X) Lock: Grants full control over the data, preventing other sessions from reading or modifying the data.
  • Intent Locks (IS, IX, etc.): Placed on a higher-level resource (like a table) to signal that a lower-level lock (like a row or page) will be taken for modification or reading.

When Does the LCK_M_IX Wait Type Occur?

  • Concurrent DML operations: If multiple sessions are trying to modify the same table, one session may be waiting for an IX lock while another session holds locks that prevent the modification (e.g., exclusive locks from a long-running update).
  • High Contention on a Table: If there are frequent read (shared locks) and write (intent exclusive/exclusive locks) operations on a table or index, contention can lead to LCK_M_IX waits.
  • Lock Escalation: If SQL Server escalates row/page locks to a table-level lock due to high concurrency, it can cause other sessions to wait for an IX lock on the entire table.

How to Identify LCK_M_IX Waits

You can identify LCK_M_IX waits using the following query:

sqlCopy codeSELECT
    session_id,
    blocking_session_id,
    wait_type,
    wait_time,
    wait_resource
FROM sys.dm_exec_requests
WHERE wait_type = 'LCK_M_IX';

The blocking_session_id column indicates the session that is blocking the current one. This can help pinpoint lock contention and resolve it by reviewing the queries being run by both the waiting and blocking sessions.

Common Causes of LCK_M_IX Waits

  1. Long-running transactions: A transaction holding locks for an extended time (e.g., a large update or delete operation) can cause other sessions to wait for IX locks.
  2. Poor Indexing: Inefficient queries that scan large portions of a table instead of using an index may increase locking contention.
  3. Frequent Read/Write Mix: When there is a high mix of read and write operations on the same table, especially with long running queries.

 

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