Comment by jpablo

  If you’re migrating a large MySQL database and you’re not 
  using mydumper/myloader, you’re doing it the hard way.

If you aren't using xtrabackup you are doing it wrong. I recently migrated a database with 2TB of data from 5.7 to 8.4 with about 15 seconds of down time. It wouldn't have been possible without xtrabackup. Mysqldumper requires a global write block, I wouldn't call blocking writes for hours a "zero downtime migration".