The original promise of RAID (Redundant Arrays of Inexpensive Disks) was that it would provide fast, reliable storage using cheap disks. The key point was cheap; yet somehow we ended up here. Why?
RAID-5 (and other data/parity schemes such as RAID-4, RAID-6, even-odd, and Row Diagonal Parity) never quite delivered on the RAID promise — and can’t — due to a fatal flaw known as the RAID-5 write hole. Whenever you update the data in a RAID stripe you must also update the parity, so that all disks XOR to zero — it’s that equation that allows you to reconstruct data when a disk fails. The problem is that there’s no way to update two or more disks atomically, so RAID stripes can become damaged during a crash or power outage.