RAID 1 is Mirroring without parity or striping.
Identical data is written simultaneously to two hard disks. This RAID level provides fault tolerance from disk errors and from the failure of a single drive. As with any storage device, it does not protect the user from his own errors, such as mass deletion, format, or data overwrite, since whatever happens on the one drive is replicated on the other.
Performance is marginally enhanced when reading data from the array, although the write performance is slightly reduced.
Disk duplexing involves having a disk controller for each drive in the array. This offers slightly better performance than mirroring, as it employs a techique called “split seek” which reads the data from the disk that can serve it more rapidly. Disk duplexing was fairly common in the early to mid 1990s when controller failures were relatively high.
