So if we assume that we can safely operate at some fixed level of redundancy (say 3 copies) across all hard drive vendors, the only question is (how many drives you need to replace per year) × (the price of those drives).
Obviously the type of redundancy used will depend on your specific application requirements, including performance.
I saw this happen once. A drive died, it was replaced and while the array was rebuilding several more failed. They ended up losing the entire SAN.
It wasn't a good day.
Of course, that doesn't work at a large scale, but then you have other methods of increasing redundancy.
As for paying for the power+drive bays, that's true of the consumer NAS devices but if you buy used servers instead it isn't such a problem. For the price of a 12 bay Synology disk station you can buy a 24 bay, 4U, used Supermciro 846E16-R1200B with a couple of CPUs and 144GB of RAM.
Not a great choice if you're short on space but can be decent otherwise.
When you're using this as the sole drive in a desktop machine or something like that, paying the extra for a HGST is pretty straightforward, but I see too many people passing over Seagate or WD when they intend to put them in a RAID, for "reliability"'s sake, without (it seems to me) much thought into whether it's really worth it.
Sorting by $/GB for Hitachi, Seagate and WD on drives >= 4TB (http://pcpartpicker.com/products/internal-hard-drive/#m=19,3...), you get:
- 4TB WD: $129
- 4TB Seagate: $114
- 4TB Hitachi (HGST): $158
So in order to get the reliability of a HGST, you're paying a 38% premium, is the extra reliability helpful in a RAID?
If you look at say a RAID6 array, aiming for usable storage of say 16TB, HGST's longest running drive (at 0.11% failure rate) would have a roughly 0.000016% chance of failing in a given year (0.0011^3 * 6 * 5 * 4) (chance of first drive failing = 6x(single), second=5x(single) etc.), assuming nobody replaced a failed disk, since you need 3 drives to fail.
Now an array of Seagate's longest running drive (2.66% failure rate) of the same size would have a chance of failure of ~0.225853% in a given year.
But, the cost of the disks in the Hitachi array would be 6x158 = $948 while the price of the disks in the Seagate array would be 6x114 = $684. For the price of the Hitachi disks you can get 8 Seagate disks.
So what happens if you just add the extra Seagate disks to the array as extra parity? Now you need 5 drive failures, giving an equation that looks like ((8 * 7 * 6 * 5 * 4 * (0.0266^5)) * 100), and a chance of failure of 0.008949%, still way higher than the Hitachi.
In the end buying Hitachi/HGST seems like the right choice anyway but I thought it interesting since I hadn't seen anyone else look at things this way.
If anyone has any problems with my math, please feel free to point it out, my stats background is pretty limited.