You can very efficiently implement MySQL database overflows. If your outline is simple, it can be done at your application level, but if it's more complicated, you might want to use a tool. Many options are described on our website here, as well as advanced options that we support.
Learn more about the options here:
http://www.dbshards.com/articles/database-sharding-whitepapers/
It is also important to consider the entire life cycle for a scanned environment, including disaster recovery, active replication, backup, recovery, and re-scalding, as you mentioned above.
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