How should I configure this server arrangement?

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http://serverfault.com – I have two Raspberry Pi's, and an old Linux box. Out of these three I want to create a reliable web server and a reliable MySQL server - however I need one of the Raspberry Pi's to be the main web server. After having done some research, I have come up with a few scenarios that would provide both of those: Primary Raspberry Pi hosting web, Second Raspberry Pi hosting MySQL, Linux box backing up both every 15 minutes. Primary Raspberry Pi hosting web, Secondary Raspberry Pi hosting backup web, Linux box load balancing (some how?) and also hosting MySQL, then periodically backing MySQL up to (HowTos)