- Feb 8, 2011
- Pacific Northwest, USA
- Real Name
I get your last three points, but I still don't understand your calculations on #1.1) twice as much actual data storage for a given number of raw terabytes.
2) highly fault tolerant of disk failures and, given a reasonable mean time to repair, the probability of a second disk failing within the MTTR window is almost zero.
3) Completely tolerant of any single disk, box, or wall wart failure. Worst case you might have to swap drives between boxes to get access to all your data.
4) Probably some speed advantage due to parallel operation where data access patterns happen to fit. Minor point, though.
In my case, one of my 2-bay boxes is used exclusively for backups from my machine and my wife's machine. I don't back that one up again to the gun safe SATA drive and I don't really care much if we miss a few day's backups. It's a joint proability thing again. we have gone literally years without needing anything from those backups, so for us to need something inside the MTTR window for a failed Synology box is near zero probability. So if my main NAS box failed, I'd just move its disks into the other box and be back on the air in a few minutes.
In a 2-bay unit, if you want redundancy your only choice is RAID 1/ SHR-1, which means that one drive is devoted to redundancy. Meaning that, assuming both drives are the same capacity, you'll get half your "raw terabytes" in actual storage. If I use two 4TB drives, I'll end up with 4TB of space.
Whereas with a 4-bay unit, I have the option of running RAID 5/ SHR-1 which still means that one drive is devoted to redundancy or RAID 6/ SHR-2 where two drives are used for redundancy. If I choose RAID 5 I can get 8TB of space using three 4TB drives or 12TB of space by using four 4TB drives. So this is more efficient once I go past two drives. If I choose RAID6/ SHR-2 (the belt and suspenders approach) I'd get 4TB of usable space using three 4TB drives or 8TB of space using four.
Only in the three drive RAID 6 example is this less efficient than the 2-bay example — but that comes with the benefit of double redundancy. And I've never suggested I would implement a three drive RAID 6 array. Beyond that specific highly inefficient use case I see no case where a 2-bay solution can be considered more efficient (in terms of usable vs. raw space) than a 4-bay solution. And once you get beyond 4-bays the benefits become even more obvious.
All that said, your solution clearly is working for you, and that's great. I understand as well that it has some other benefits in terms of non drive related failure. But I just don't see how you can contend that a dual 2-bay NAS solution can be considered to have "twice as much actual data storage for a given number of raw terabytes" compared to a single 4-bay solution with similar redundancy.