Moved a table with about twenty million rows last month and learned three things the hard way.
Do the double-write before the backfill, not after. We did it the other way and spent two evenings reconciling.
Measure replication lag before you start, not during. Ours spiked and we had no baseline.
Write the rollback plan first. We had one, sort of, in someone's head. That is not a plan.
