With mirroring, the system writes the data simultaneously to separate drives or arrays.
The advantage of mirroring are minimal downtime, simple data recovery, and increased performance in reading from the disk.
The disadvantage of mirroring is that both drives or disk arrays are processing in the writing to disks function, which can hinder system performance.
Mirroring has a high fault tolerance and can be implemented either through a hardware RAID controller or through the operating system. Since it requires twice the disk space than actual data, mirroring is the less cost-efficient data redundancy strategy.
Source: SWANSON, Marianne, & al., National Institute of Standards and Technology (NIST), NIST Special Publication 800-34, Contingency Planning Guide for Information Technology Systems, December 2001 (page 45).