Recommended networked file system for web servers?

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http://serverfault.com – If I have two web servers, what would the best way to keep their files in sync? Unfortunately the sites themselves cannot be altered significantly. Most of them are Wordpress based, but there are over 200 of them. There are also some custom sites. This setup is going from Web server + DB Server to Varnish load balancer + 2x web servers + DB Server. Since Varnish can send requests to either server on a whim, anything like rsync that has more than 10 secs or so lag is out of the question. There is a dedicated private switch for all the servers to use. I'm thinking against NFS where there is o (HowTos)