With respect to the tradeoff between redundancy vs. data protection, what recommendations can you make on RAID levels? Is it completely dependent on the environment, or are there clear advantages to using a RAID 5 over RAID 3, etc.?
Redundancy = data protection = availability = operational uptime - think 24/7 operation. The premise is, if a drive drops out, it will still run (i.e. RAID 5). But if a second drive goes, the array is lost, and the system is "broken", and needs the array rebuilt and restored from backups. Other levels (i.e. 10 or 6) allows for 2 drives to fail, and still remain operational (though once a disk fails in a redundant array, it's performance is degraded). In such a case, it takes a 3rd drive failure to trash the system.
RAID levels not only deal with redundancy, but have performance penalties as well as capacity differences. For example, in a level 10 array, you get 2 drive redundancy (4x disks total), but at the cost of only being able to use half of the capacity (it's performance is decent).
So the compromise consists of redundancy, performance, and usable capacity.
I'd recommend reading the
RAID wiki page to get an understanding of what each level does. Also pay attention to the write hole issue associated with parity based arrays (5/6/50/60), as only a proper RAID card (processor + cache) should be used, as it has a solution that deals with it (NVRAM).
