Mirroring uses multiple copies of all data by stripping file sectors across disks and servers. In two-way mirroring, the two copies of a file sector are written to different physical hardware (different drives in different servers) that are presumed to be unlikely to fail together. This protection method prevents data loss in the event of a single drive or single-server failure. Microsoft recommends three-way clustering for clusters with more than two nodes when using mirroring.
Three-way mirroring requires at least three servers to define three independent hardware fault domains. Because there are three fault domains, the system can tolerate at least two simultaneous hardware problems (drive or server). For example, during the reboot of one server, if another drive or server fails, all data remains safe and continuously accessible. Since three-way mirroring writes three copies of everything, its storage efficiency is 33.3 percent. Writing 1 TB of data will use at least 3 TB of physical storage pool capacity.