Unlike most RAID parity which starts with XOR bits, UnRAID uses something called even<\/em> parity. Essentially, this is a bit that when added to the sum of bits in the same location across all the drives in the array, makes said sum an even number.<\/p>\n

For instance, if the bit on Drive A is a 1, the bit on Drive B is 1, and the bit on Drive C is 1, adding them together makes 3. That being an odd number, the parity bit will be made 1 to make the result 4. This can work across any number of drives and protect against the loss of one.<\/p>\n

E.g., 1 + 1 + 0 + 1 + 1 + 0 = 4, parity 0.<\/p>\n

E.g., 0 + 1 + 0 + 1 + 1 + 0 = 3, parity 1.<\/p>\n

When a drive fails, the parity bit is added to the remaining bits. If the result is odd, then the missing bit must’ve been a 1. If the result is even, then the missing bit must’ve been a 0.<\/p>\n

E.g., 1 + 1 + 0 + 1 + 1 + (P)0 = 4, missing bit 0.<\/p>\n

E.g., 1 + 1 + 0 + 1 + 1 + (P)1 = 5, missing bit 1.<\/p>\n