RAID 1 protects data against a drive failure and allows some increase in read
performance. Data is protected by duplicating the contents of each drive on a second
drive in the set, a process called mirroring. Because of the duplication, a volume based
on a RAID 1 set can’t be larger than half of the total space available on the drives in the
set. You can create a RAID 1 set using either two or four disks.