Use RAID 1 (mirrors) volumes when you need fault tolerance because
even a disk failure will not interrupt service. RAID 1 volumes can
withstand multiple disk failures, depending on which disks
actually fail. Some of the most popular uses for RAID 1 (mirrors)
volumes are:
Creating mirrors for the root filesystem (and /var, and /usr)
Duplicating disks for system integrators (that is, create a mirror for the root drive to a new drive, allow the resync to finish, then move the new drive to an identical system for immediate use)
Providing an offline copy of data to allow a longer backup window. For example, a three-way mirror could have one submirror offlined (then attached to a different system for backup purposes) while the remaining two sides of the mirror continue to provide data redundancy.