A RAID-1 or mirrored volume is used to create two identical copies of a volume. The amount of space allocated to the volume on each disk must be identical, and data is written to both disks through a technique called mirroring (if one disk controller is involved) or duplexing (if two disk controllers are involved). With standard mirroring, two independent writes are needed one to each disk which affects write performance. With duplexing, the two writes can be performed simultaneously using separate disk controllers, so write performance is the same as if one disk was being used.
Mirrored volumes provide fault tolerance. If one of the drives in the mirror set fails, the other disk can continue to operate and there is no data loss. The failed drive must be replaced or another drive must be specified as the mirror partner to restore fault tolerance.