Backend & systems·Data modelling and storage engines
you added a column and now have to populate it for eighty million existing rows without locking the table or saturating the disk.
Backfill
Populating a new field or table for existing data, usually in batches with a delay between them. Doing it in one statement takes a long lock and a large amount of replication lag with it. The batch job also has to handle rows written after it started, which is why backfills are normally paired with dual writes.