Servers may have hardware or other limitations of two disks that can be used for the system.
In a two-disk configuration, the system disk is mirrored on a second identical disk. Both disks are bootable.
Although it is usually taken for granted that disks are mirrored in a two-disk configuration, we want to stress that this is not the best choice in all cases. For example, consider a system that is used as a JumpStart_ technology server where continuous availability of the server is less important than protecting the configuration files that exist on this system. From this perspective, using the second disk as a contingency disk may be a better choice than an on-line mirror.