I'd suggest configuration like :
1. load balancer + n app servers + 1 db server
app servers should have one shared folder for data dir ( on some nas , san or simmilar), mounted with rw permissions and SAME user, on all of them, and then you put that folder as data_dir in config file...
that way you have balanced load on app servers, and all the data files accesible to all servers (via single data dir )
data_dir is new config option for v8 ( this trick will not work in v7 or eralier version) - folder contains all session files , as well as all attachments and downloaded modules per database...
This is easier version, more sophisticated includes also n db_servers wich should then be carefuly replicated and synced..
Hope this will help a bit..
Any Solution?