Well what I'm wondering is, say I found a tower that could house the cards, and I had a RAID controller plugged into the mobo inside. How would I hook the drives up to the RAID card? Do RAID cards have tons of ports? Or is there some other device I'd need to use into which I'd plug all the disks, that then connects to the RAID card via a SAS cable?
Performance requirement: should be able to pump 100 megs/second. That's plenty to have half a dozen (at most) people streaming compressed bluray content (that's in the neighborhood of 10 gigs tops per 7200'ish seconds which is only 1.5 MB/second) or mounting ISO's off of it or whatever. And reasonable for writes as well. And fast enough for rebuilds to not take forever in case of drive failure. I'm pretty confident that with 12 drives in any RAID config, I'm going to be able to get that bare minimum 100 megs/second, so I'm not worried about performance at all.
Fault tolerance requirement: I was considering doing a RAID 60 with two six-drive layers so it could absorb 4 disk failures as long as no more than two were from a given layer. But I believe RAID 6 will be sufficient, since it takes me from 2/3 space utilization to 5/6 and I'll have spare drives handy to deal with failures.
So what I know I want is: 12x4TB drives in a RAID 6 configuration.
What I'm still trying to decide is: what type of enclosure/NAS/computer-with-really-big-tower-that-can-house-many-drives/RAID-card/etc. do I want. So far enclosure seems to be the single most expensive component (all drives together being more expensive of course) and I figured maybe there's no real need for an enclosure as long as I've got a big tower that can house 12 drives all hooked up to a RAID card somehow. Just wondering how exactly they hook up or if I'm misunderstanding how RAID enclosures work. Having used the Mediasonic which handles everything, including RAID, means I've never had to actually construct a RAID array by hand before.