Table partitioning migration with dual writes and backfill creates sustained MultiXact pressure

Migrating from monolithic table (~10TB) to partitioned structure with dual writes and backfill creates sustained write pressure. Backfill processes with foreign key relationships rapidly consume MultiXact member space, especially when combined with other scheduled write operations (e.g., monthly invoice generation). Scale of 30TB+ database means vacuums take hours.

Rethink backfill execution strategy to avoid thrash on online datastores. Lower backfill concurrency and monitor MultiXact member space consumption continuously. Avoid scheduling other high-write operations (like invoice generation) during backfill periods. Consider staged migration with vacuum windows between phases.