Migrating a giant production database without downtime
I don't know if this is possible with the hosting options you are using, but I would tackle the problem this way:
- Copy the data to the new server (and record the binary log position on the old database server)
- Turn on replication from the old server to the new server (and wait for it to catch up)
- Reconfigure the Rails application to point to the new server
Read only mode is another potential option if a "write downtime" is more acceptable than a "full downtime" (depends on the application obviously).