Also been there. Percona Toolkit works fine (though it's not load-aware enough, so we changed the backfill logic). Plus, in 5.7, there are a fair number of on

Also been there. Percona Toolkit works fine (though it's not load-aware enough, so we changed the backfill logic). Plus, in 5.7, there are a fair number of online-DDL changes, and adding indexes is (usually) one of them.