Online ALTER COLUMN final Sch-M lock duration bounds under concurrent DML in SQL Server 2019

Context

SQL Server 2016 introduced online ALTER COLUMN for certain data type changes, allowing concurrent reads and writes during the operation. For incompatible conversions (e.g., int to bigint), a data-copy phase occurs followed by a brief schema modification (Sch-M) lock to switch metadata.

Unresolved behavior

The documentation does not specify a maximum duration for the final Sch-M lock acquisition. The wait depends on transaction log flush, checkpoint activity, and any long-running transactions holding shared locks. Under heavy write load, this lock can extend to seconds, causing application timeouts — a gap for strict zero-downtime requirements.

Constraints

• No built-in MAX_DURATION or timeout option exists for ALTER COLUMN (unlike resumable index rebuild).
• Edition support: Enterprise/Developer in 2016-2017; Standard from 2019 CU1+.
• No DMV exposes real-time progress of the data-copy phase.

Questions

1. Is there any documented upper bound or predictable worst-case for the final Sch-M lock duration during online ALTER COLUMN?
2. Can the lock acquisition be programmatically bounded or aborted without third-party tools?
3. What are the supported workarounds for guaranteeing zero-downtime schema changes when the Sch-M lock wait is unacceptable?