Last time I checked, nearly every database worth using on a busy site supported some sort of real-time replication and/or live migration. If that doesn't work,

Last time I checked, nearly every database worth using on a busy site supported some sort of real-time replication and/or live migration. If that doesn't work, there's always the backup plan: say the magic words "scheduled maintenance", service $database stop, rsync it over, and bring it back up. The sky will not fall if HN goes down for another couple of hours, especially if it's scheduled ahead. :)