IMO a RAID 6 for an FTP server is quite overkill. RAID 5 will do just fine. Do note that 100% reliability and availability cannot be guaranteed unless you actually make the whole chain redundant: from ISP to routers, switches and actual servers, AKA the entire hardware chain. The question you'd have to ask yourself is: "how much am I willing to spend for what level of availability?" Unless you're in a corporate environment where a simple FTP server is absolutely mission-critical, most will not accept the cost of 100% failover for what is essentially a simple service. And even if you're in a situation where said FTP service is mission-critical, my advise would be to rethink that strategy to become less dependant on a single point of failure...
You do realise that 15 TB is quite a lot, and so is 12 disks in a RAID. Dunno what you're planning to use it for (downloading the internet perhaps?
) but my advise would be to start significantly smaller, in a RAID 5 setup, with 3 disks and a single hot-spare with LVM on top. Configure the array thus that the remaining disks are also spares, then as time and diskspace in use dictate, add more storage to the LVM after adding the corresponding disk to the array. Webmin does a good job of managing RAID, no experience with LVM though...
