You can use gh-ost (or most other online schema change tools) to get around that. You'll still have some necessary increase in write load since you are double-w

You can use gh-ost (or most other online schema change tools) to get around that. You'll still have some necessary increase in write load since you are double-writing that table but you can control the pacing of the data backfill (gh-ost gives even more control as it doesn't use triggers to duplicate writes). It's not quite as simple as just running ALTER TABLE on the master though.