RAID 1 known also as mirroring provides fault tolerance from disc errors and failure of all but one discs. Data is duplicated on two different drives either a hardware RAID controller or software. Duplexing in RAID 1 provides tolerance against failure of either a drive or a controller. One of the main advantage of this RAID level is that data recovery takes less time than in e.g. RAID 0 level, matrix rebuilding generally is limited to replacing faulty drive.

Controller Requirements: supported by all types of controllers and software solutions
Hard Disk Requirements: two drives, drives should be identical
Array Capacity: size of the smallest drive
Storage Efficiency: if drives are the same size 50%
Fault tolerance: very good, improved by using duplexing variant
Availability: very good, most RAID controllers support hot sparing and automatic rebuilding
Degradation and rebuilding: read performance a little degraded, write performance improved
Random read performance: good, worse than many other RAID levels but better then single drive
Random write performance: good, better than many other RIAD levels but worse than single drive
Sequential read performance: more or less the same as single drive
Sequential write performance: good, better than many other RAID levels