Online MySQL Migrations with Dual-Write Cutovers

Schema migrations on multi-terabyte datasets are operational programs, not single SQL statements.

Safe Sequence

1. Shadow table creation.
2. Backfill in bounded chunks.
3. Enable dual-write.
4. Validate checksums continuously.
5. Gradual read cutover, then write cutover.

INSERT INTO users_v2 (id, email)
SELECT id, email FROM users
WHERE id > ? AND id <= ?;

No cutover without sustained zero drift.