RAID 6 with four 8 TB drives: double parity in the smallest array

RAID 10 on the same four drives gives the same 16 TB, so capacity doesn't separate the two. The cost of RAID 6 lands on writes instead. Each write updates two parity blocks per stripe where RAID 5 updates one, so writes are slower, and that gap is built into the layout.

Against RAID 10, the difference is the second failure. RAID 10 on four drives guarantees 1 failure. After one drive dies, a second failure only spares the array if it lands in the other mirror pair, which is two of the three drives that remain. That's 66.7% of the time when the second failure picks a remaining drive at random. RAID 6 survives any second failure.

A single failure on this array reads the three survivors, 24 TB. On a quiet array, one 8 TB drive takes about 14 hours at best. The second parity block means a second failure during that window still leaves the data intact.

Four bays is the floor, and each extra drive lowers the parity share. Six drives bring the overhead down to 33%, and eight drives bring it to 25%.

RAID 5 on three of these drives gives the same 16 TB, but it survives one failure where this layout survives two.