XRAID Capacity / Redundancy with ReadyNAS Ultra 6 Plus

FoxyWFU wrote:
If I start with 2tb drives in bays 1 & 2, this will give me the same capacity & redundancy I currently have.

Roughly, yes.

FoxyWFU wrote:
What happens if I add a 2tb drive in bay 3? Does that give me double the capacity?

Using X-RAID2, yes. If any one disk fails data remains intact. See X-RAID2 Single-Redundancy in Action

FoxyWFU wrote:
What data is redundant if XRAID considers every drive as one volume? Would I need to add a 2tb drive in bay 4 to make drive 3 redundant?

When we talk about volumes and the ReadyNAS we are typically not talking about physical volumes (i.e. hard disks / hard disk partitions), but rather logical volumes such as an X-RAID2 volume.

Here's an explanation that might help.

Scenario A

We have 2x2TB disks, disk A and B and ReadyNAS is redundant
We add a 3rd 2TB disk disk C, a resync occurs and redundancy is restored.

Considering the sectors of the disks, they are synced so that:
1/2 B + 1/2 C = A
1/2 A + 1/2 C = B
1/2 A + 1/2 B = C

The volume is redundant

We add another 2TB disk D and redundancy is again restored so that

1/3 B + 1/3 C + 1/3 D = A
1/3 A + 1/3 C + 1/3 D = B
1/3 A + 1/3 B + 1/3 D = C
1/3 A + 1/3 B + 1/3 C = D

Scenario B

We have 2x2TB disks and add a 3TB disk.
Only 2TB of the 3TB drive can be made redundant so the 3TB disk is treated like a 2TB disk. However if we add a second 3TB disk we can now use that space, getting two layers, 4x2TB RAID-5 + 2x1TB RAID-1


All the above is talking about X-RAID2 single-redundancy.

Personally I recommend X-RAID2 dual-redundancy (requires at least four disks) so that if any two disks fail data remains intact.

Hope this helps.