Вот здесь есть некая волшебная штука - SQL Relay
Обещают возможность соединения с множеством MySQL серверов
The good news is that SQL Relay’s performance here is comparable to PHP’s persistent connections. The reason is because SQL Relay has already established X number of Oracle (or MySQL) connections and strives to maintain that number continually. Then, your PHP app connects and disconnects from the SQL Relay server, which is lightweight.
Getting everything to run smoothly does take a fair bit of tuning and careful management of your SQL Relay connections. If you have leaky code that doesn’t disconnect properly, you can still flood your connection pool, no pun intended. Wish I had benchmarks but, the main thing is that SQL Relay offers similar performance and won’t flood your database with connections, both of which are good things.
Выглядит заменчиво, надо посмотреть - вероятно это дешевая альтернатива domain data separation при больших нагрузках
