I currently have 2 disks in my RN104 configured in raid 1, so basically 1 mirrors the other.
My question is, what will happen if I fill the 2 remaining slots. Will both extra disks be mirrors of the first 2, or will they form their own mirror pair?
I guess you can do both if you switch to standard raid, but the default for x-raid2 is to configure them in raid5. Raid 5 tolerates one failed disk at a time but you get the space of n-1 disks, so with four disk of identical size you get 3*size, the last disk is used for parity. (actually the parity disk should change each time you copy but it's simpler saying one is reserved).
There is no separate parity disk, the parity blocks are evenly distributed across all the disks. Distributing the parity blocks evens disk usage and improves RAID write performance.
Other than that minor detail, xeltros is correct. The default xraid2 will automatically switch to RAID-5 when the third disk is inserted.
