Category: Wait Types

LCK_M_SIX is a wait to acquire a Shared With Intent Exclusive (SIX) lock. SIX is a combination lock: the session wants to read the whole table (shared) while also modifying some rows within it (intent exclusive). SQL Server uses it for scan-then-update patterns, where a query reads broadly but writes selectively. You will see this…

LCK_M_SCH_S is a wait to acquire a Schema Stability (SCH-S) lock. Every query takes SCH-S locks on the objects it touches, during compilation and execution, to guarantee the table’s structure does not change underneath it. SCH-S is the most compatible lock in the engine; it coexists with everything except one thing: a Schema Modification (SCH-M)…

LCK_M_SCH_M is a wait to acquire a Schema Modification (SCH-M) lock. DDL needs this lock to change an object’s structure: ALTER TABLE, offline index rebuilds, partition switches, truncates, and drops all take SCH-M. It is the least compatible lock in the engine; it conflicts with every other lock, including the Schema Stability locks that every…

LCK_M_RX_X is a wait to acquire an Exclusive Range lock: an exclusive lock on a key value plus an exclusive lock on the range between it and the previous key. Like all range locks it only exists under SERIALIZABLE isolation, where writers must lock the gaps between keys so no other transaction can insert a…

LCK_M_RS_U is a wait to acquire an Update lock on a key together with an Update Range lock on the gap between it and the previous key. It belongs to the serializable range-lock family: SQL Server takes it when a serializable transaction searches a range it intends to update, using update-mode semantics to stay deadlock-friendlier…

LCK_M_RS_S is a wait to acquire a Range Shared lock: a shared lock on a key value plus a shared lock on the range between it and the previous key. Range locks only exist under SERIALIZABLE isolation, where SQL Server must prevent phantom rows from appearing in a range a transaction has read. Seeing this…

LCK_M_RIn_NL is a wait to acquire an Insert Range lock with a NULL lock on the key itself. It appears when a session under SERIALIZABLE isolation wants to insert a new key: SQL Server locks the range between the neighbouring keys to keep the insert phantom-safe, while the NULL component on the key is an…

Every lock mode in SQL Server has two extra wait type variants: LCK_M_<mode>_LOW_PRIORITY and LCK_M_<mode>_ABORT_BLOCKERS, 46 wait types in all (LCK_M_S_LOW_PRIORITY, LCK_M_SCH_M_ABORT_BLOCKERS, and so on). They exist for the WAIT_AT_LOW_PRIORITY option on online index operations and partition switches (SQL Server 2014 onward), which lets DDL wait politely instead of queuing the whole world behind its…

LCK_M_IX is a wait to acquire an Intent Exclusive (IX) lock. When SQL Server modifies a row, it first stamps IX locks on the page and table above it, signalling that exclusive locks exist lower down. This wait means a writer cannot even place that intent marker, because another session holds an incompatible lock at…

LCK_M_IS is a wait to acquire an Intent Shared (IS) lock. Before SQL Server takes a shared lock on a row or page, it places IS locks on the table and page above it, marking its intention so other sessions know readers are active lower down. This wait means even that lightweight intent marker is…