RAID 1

(Redundant Array of Independent Disks Mode 1) A popular disk or solid state drive (SSD) subsystem that increases safety by writing the same data on two drives. Called "mirroring," RAID 1 does not increase performance. However, if one drive fails, the second drive is used, and the failed drive is manually replaced. After replacement, the RAID controller duplicates the contents of the working drive onto the new one. See RAID 10 and RAID.

Mirroring for Fault Tolerance

Widely used, mirroring writes two drives at the same time so that data are duplicated. It provides the highest reliability, but doubles the number of drives needed.

RAID 1 Probability of Failure
The more drives in a RAID 1 array, the lower the probability of failure. For example, if experience tells us that one out of a thousand drives fails in a year, the probability that an entire 2-drive array will fail in a year is 1 in a million; that an entire 3-drive array will fail is 1 in a billion and so on.

The formula: if the probability of failure of each of n drives is p, then the probability that all the drives will fail is p^n.