ReadyNAS Ultra 6 Plus.... 18TB -> 24TB

frusnet wrote:

I am not very experienced in these NAS / RAID configurations at all, but I think I had chosen dual redundancy. When I set it up first, out of 6x3tb = 18TB space, I had about 12TB space left for my files. 2 disks's space was kind of reserved for keeping the NAS running in case of failure. That 12TB "usable" space is what I intended to upgrade, and therefore I got 6x4TB's for that reason. One side note... a conflicting log message with above... A disk was removed from the ReadyNAS. One or more RAID volumes are currently unprotected, and an additional disk failure or removal may result in data loss. Please add a replacement disk as soon as possible. Doesn't this conflict with dual redundancy? Maybe the X-Raid2 option I chose was single redundancy?? (then how come I had 12tb usable space out of 18TB's?)

The message you ask about applies to single redundancy. You removed one disk, and the volume is intact but not protected. That's single redundancy. If you had dual, then the volume would still be protected.

Single redundancy is the xraid2 default. You would have needed to explicitly set it up for dual redundancy back when you initially set it up. It's pretty clear at this point that (a) either you didn't do that at all, or (b) you tried to do it, but somehow ended up with single redundancy anyway.

One complication on total space is that there are two sets of units floating around. For historical reasons, drive manufacturers use multiples of 10 (1000 bytes = 1 KB, 1000 000 bytes = 1 MB). Windows and many other OS use multiples of 1024 (1024 bytes = 1 KB, 1024*1024 bytes = 1 MB, etc). These "power-of-two" units are technically called KiB, MiB, GiB, TiB, but Netgear (and Windows) leave out the middle "i'. This didn't matter much when 20 MB was a large disk, but as capacities grow the differences between the units becomes substantial.

6x3TB gives you 15 TB (using power of ten), but only 13.6 TiB (using 1024 multiples). The NAS should have reported 13, not 12 - but you might not have noticed the free space until after you had already copied some stuff.

frusnet wrote:

Let me summarize basically what I need, maybe it's easier to find a conclusion - I need more space than 12TB... whole reason of getting 6x4TB disks. After devoting 2 disks to safety I expect to have 16TB usable space (at least). - Since I'm using "desktop" level drives, I will need the NAS to be up and running even 2 drives fail at the same time. I can live with 1 drive failing too but I think it's too much of a risk. - Which RAID level should be optimum based on above with 6x4TB disks?

Two main points.

One is the TiB/TB thing again. 6x3TB single redundancy is 15 TB (13.6 TiB). 6x4TB dual redundancy is 16 TB (14.5 TiB). By going to dual redundancy, you are getting extra protection, but only 1 TB of extra space.

Dual redundancy is better than single. But backups are what are really needed in either case. RAID gives you two benefits, and they are in the convenience category not so much vital protection. The first is that if you have a routine disk failure, you can pop in a new one - keeping your data available, and not going through the bother of a restore from backup. The second is that you can expand the volume (without destroying it, creating a larger one, and then restoring the data). In your case, since you've hit the expansion ceiling, so you only get the first benefit.

Some people (including some posting on your thread) think dual-redundancy is the way to go with larger disks. Personally I think single redundancy is fine, as long as you have backups. I haven't seen any evidence that larger disks fail more frequently than smaller ones. But it is obviously true that it is more painful to restore a bigger volume when it does fail. So there is no right answer here. RAID-6 (same as "dual redundancy xraid2") offers more protection from drive failures, at a cost of losing more capacity. For some people, that's a price they are willing to pay. For others, it isn't.

frusnet wrote:
- In the future, ability to upgrade to 6x6TB maybe?

I believe you can do this with flexraid, by creating a second 6x2TB volume to use the extra space. Though I haven't tried to do that myself. OS6 xraid2 would just expand, as it doesn't have the 16 TiB ceiling (because it uses the btrfs file system).

frusnet wrote:
- In a nutshell pros/cons of OS6

It's newer, and has some better features (more useful snapshots and antivirus protection are probably most important to you). It has no expansion limits.

The bad news is that Netgear support won't help you if you have issues, since Netgear hasn't released OS6 for the Ultra-6. The hardware warranty still applies of course. The in-between news is that OS6 is still a bit of a work-in-progress. Though the recent releases have resolved most of the growing pains.

If you want to move into OS6, then now would be a good time to do it - since you need to rebuild the NAS from scratch anyway. You can reverse the process, but that also requires rebuilding the NAS from scratch. There's a quite long thread on OS6 for legacy devices, you should browse through it if you are interested.

frusnet wrote:
- How to do off-line expansion process?

Given the discovery that you actually only have single redundancy, this process won't work for you anyway. So don't worry about it.

frusnet wrote:
- "If you decide to go with the factory default, you might as well go with flexraid RAID6 - not much point to xraid2 if you are at the 16 TB ceiling. " This 16TB ceiling is for x-RAID2? Basically which Raid configuration you think would best serve above criteria.

The 16 TiB ceiling is for an ext4 volume with 32 bit inode addressing. The NAS sets up the file system to use 32 bit addressing by default, unless the original raid volume size is > 16 TiB. Once done, there is no safe way to expand the inodes to 64 bits. That 32-bit decision made sense when it was made (back when 2 TB disks were the biggest you could buy), and it still works out well for the 4-bay units. But with 4 TB and larger drives available today, it isn't the best.

It'd be nice if Netgear made a fairly small mod to always create a 64 bit inode volume. There are several use cases where that would be helpful to the installed base of x86 users.