Why don't we use RAID 8?

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All RAID types are built using three elements:
  1. Striping Data blocks on several disks.
  2. Writing redundant data (it is usually the result of calculating particular functions).
  3. Writing multiple copies of data (usually two copies are written).
Different combinations of these elements allow to get data placement patters (RAID levels) which provide desired balance between speed, reliability and price.

If we only use striping, we get a RAID 0. Using only one function to calculate redundant data we will get us a RAID 5. If we add one more set of redundant data to a RAID 5, it becomes a RAID 6. We get a RAID 1 from identical copies, but if you combine the striping technique with exact copies then you get a RAID 10. These data placement patterns are the most widespread and well-known, forming a so-called raid triangle.

Except these RAID types, we can also see exotic combinations.

For example, if in RAID 10 we use the number of disks which is not an integral multiple of the number of data copies we will get significantly less symmetrical array called RAID 1E. Also in RAID 1 we can use three copies of data instead of two. Fault tolerance increases and the price of the device increases as well. The same way, if you add the third set of redundant data to a RAID 6, fault tolerance increases even more along with the price and performance decreases.

Exotic RAID types are rarely used because they have too much imbalance between price, speed and fault tolerance. RAID 6 is an still acceptable tradeoff, but RAID 8 is not.

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