RAID redundant array of independent (or inexpensive) disks A RAID provides convenient, low-cost, and highly reliable storage by saving data on more than one disk simultaneously. At its simplest, a RAID-1 array consists of two drives that store identical information. If one drive goes down, the other continues to work, resulting in no downtime for users. RAID-1 isn't a very efficient way to store data, however. To save disk space, RAID-3, -4, and -5 "stripe" data and parity information across multiple drives (RAID-3 and -4 store all parity data on a single drive). If a single disk fails, the parity information can be used to rebuild the lost data. Unfortunately, there is a performance trade-off: depending on the RAID type used, a RAID will be slower than a single drive at either reading or writing data.