High availability storage -2 cases. What do you use/suggest?

[manwe_]

Hi *.

I need to work on high availability of my web servers (running Gentoo of course). Some topics I have already covered (like Percona Xtradb Cluster, nginx rev. proxy, etc.). Last "big" subject for me is file storage for PHP applications. There are two separate cases I'll try to describe. I'd like to ask what solutions do you suggest, what are your experiences with them, what to avoid, etc. I have my types, but before digging into one solution and finding myself neck deep in shit, I'd like to hear from with people with better experience

First case is 15 servers with thousands of virtual hosts. Just PHP + files, not accessible from outside (nginx proxy on front servers). So far 6TB of data on them. Less than 500 write ops per server (3k in peaks), almost no read ops (128GB RAM, ≥60% of it is used by file system cache). I'd like to combine that storage to one, accessible from all the servers simultaneously. This way I'd be able to lost even 5 or 6 machines and still be able to serve everything.

Second case is just one server with ~50GB of data in 10 PHP sites. Nothing big, but those apps are quite important. So I've set up second, identical server (empty for now). Now I need to mix them together. I see two ways of doing this. Split sites (5 on first, 5 on second) or all sites on both servers and split traffic. Nevertheless, files need to be in sync in case one server crashes and second needs to do everything.

What's your opinion?