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Steve 46 wrote:
Regarding X-RAID, I understand that is the default single drive failure protection configuration. So if I have four drives loaded and one fails I would still retain all my data? I also understand that one drive is for parity information but not really understanding how one drive can protect the information on the three other drives. What if the parity drive fails?

As mdgm explains, xraid-2 distributes the parity blocks. Though even if it did not, the parity blocks can be recomputed from the data blocks, as long as the data blocks are intact. Distributing parity blocks just evens out the reads and writes across the drives (which helps performance, and evens drive wear).

More details if you want them: The 4-drive raid array is organized as horizontal stripes, with each stripe containing one parity block and three data blocks

block 1, block 2, block 3, parity
block 5, block 6, parity, block 4
block 9, parity, block 7, block 8
parity, block 10, block 11, block 12

The way the parity works is that

block 1 XOR block 2 XOR block 3 XOR parity = 0

XOR is an "exclusive or" operation, but you can envision it as a strange form of addition.

block 1 + block 2 + block 3 + parity = 0

When a disk fails (say disk 2), the NAS can compute the missing block by solving the equation above.
block 2 = 0 - block1 - block3 - parity

With XOR, this turns out to simplify to
block 2 = block1 XOR block3 XOR parity

The way this is set up, no matter which disk fails, the disk is recovered by XORing the other three.

With RAID-6 (dual redundancy) the math is more complicated, and amounts to solving 2 simultaneous equations. Each stripe has a second "parity" block sometimes called Q, which is computed using from the data blocks using a different equation. With two drive failures, you end up with two equations with two unknowns - which the NAS has to solve to recover the disks.