A mirror only lets you use one disk in a stack in usable space (50%, or less if for some reason you want a double redundant mirror, i.e. three disks). A RAID-Z depends on the amount of parity disks.

Since the dawn of computing, long-term mass storage has been a primary factor in the design of systems. At issue is speed, density of storage, and of course, fault recovery.

The smallest RAID arrays are 4 drives, but 6-8 drives in a RAID stripe is most common. With RAID 5, one drive's worth of capacity is dedicated to parity, while in RAID 6 two drive's worth are.

Broadly speaking, however, when an all-flash array is a legacy product that’s been retrofitted with flash capacity then you find Raid levels that span the possible combinations of mirroring ...