6.2.0 upgrade problems on RN104

Another way to understand it is that there is an equation set up by the parity block and the associated data blocks. For each byte in the data blocks, DB1+DB2+DB3+PB1= 0. The "+" is really XOR, but it functions like normal addition (in a Galois field, which is the magical part. It also functions as subtraction in the same field, which is not intuitive at all). Anyway, if you know that one piece is missing (say DB3) you can solve the equation (DB3 = 0-DB1-DB2-DB3-PB1), and recover the lost data. With RAID-6 you have two different equations, and you can similarly solve them. Since there are two equations, you can solve for 2 unknowns.

The "magic" part is the modern algebra (the Galois field bit). Modern Error correcting mechanisms and loss recovery mechanisms all depend on modern algebra. But if you translate to ordinary algebra, you can understand the gist.

One constraint is that you need to know up front what piece of data is missing. In normal RAID that is determined by a read failure of one of the blocks.

Cheers, I think I'm just about grasping it now! I also realised I'll gain 6TB not 4TB, so perhaps it will be worth it after all. Time for bed now, and maybe I'll have made up my mind by this time tomorrow which mode to use...

Babbage wrote:

I've been reading the manual again and giving some thought to which RAID mode to use, and it occurs to me that X-RAID2 might be a better choice as it would allow me to gradually upgrade to 6TB drives over the next 4 months rather than buying 4 all at once and having to restore all the data yet again. However, with the existing 4 x 3TB drives it will mean 3TB of disk space will be reserved for data protection, which I would have to cope with by temporarily keeping the double backup, and I would only end up increasing my total storage space by 4TB, so I'm not sure if this is going to be worth it.

To respond to your practical question:

-4x6TB xraid2 won't work on your RN104. There's a 16 TiB volume limit on the RN100 series (flexraid or xraid), and that configuration is 18 TB. You can go to 2x6TB+2x5TB, but that is the ceiling. Also, the biggest drives on the hardware compatibility list are 4 TB. Netgear will deny support (perhaps even paid support) if you stray from the HCL.

-A second RN104 with 3x4TB drives is actually a cheaper way to get 6 TB more storage (actually it gives you 8 TB). 4x6TB drives cost about $1000 at the moment. A second RN104 with 3x4TB would cost about $750. This is often the case when you are expanding, because you are discarding perfectly good drives. I'm not saying your path is the wrong one for you, just that there are cheaper options you might want to consider.

-RAID might not be worth it to you. Even with RAID you need to have a backup - if you follow the forum here for any length of time, you'll see there is always at least one poster learning that lesson the hard way. Personally I keep 3 copies of everything on different devices, because I've sometimes found that a backup I was depending on couldn't be read.

So, it is reasonable to start with the question "what is RAID actually good for, if you need backups anyway".

The main benefit is that your data remains available while you are recovering from a single disk failure. Without RAID, you'd lose access to the data on the disk until you replace it with a new one, and restore it from backup. You generally also lose some of the newest data, because it hadn't been backed up yet. In a business environment, losing access to your data costs you time and money and reduces your ability to satisfy your customers. So it is definitely valuable there. In a home environment, it is annoying to lose access for a few days, but often not critical. You might not be able to enjoy your media library for a week every now and then. You might need to defer doing your taxes, or some other task like that (depending on the timing). But often it is more a convenience than a necessity.

A secondary benefit is that your data is all consolidated onto one volume, which you can grow over time. You don't have a lot of places you need to search for it. You can reduce the possibility of multiple copies, some of which might be out of date. It is easier to back up because it is in one place, and it is generally easier to manage.

Is that worth the extra disk drive? For me the answer is "yes" for my primary NAS (a pro-6). On my backup NAS (more than one, smaller than the pro-6) I've mostly switched to jbod - so I guess my answer for those backup units is "not really".

Wow, thanks for explaining all that! I'm glad I checked back here before doing anything with the NAS this evening, and indeed before buying any 6TB drives. I might upgrade to 4TB drives eventually, but for now I'm going to stick with the 3TB drives I've got and use RAID-0 so that I can lump everything together for convenience instead of splitting it up across four drives.

The test-fill of drive 4 worked fine so I've wiped it again and set up a 10.8TB volume using all 4 drives. Now to fill it all up again... By chance, I just bought a cheap little PC off ebay so can leave that running 24/7 copying from the D-Link to the RN104.

Thanks again for your help.