Windows NT-based disk mirroring and duplexing implement RAID features in software, using any vendor's hardware that is compatible with the operating system. Because these are software-based solutions provided with the operating system, they have a cost advantage.
Disk mirroring protects against media failure by maintaining a fully redundant copy of a partition on another disk. This provides protection from the downtime and expense involved in recovering lost data and in restoring data from a backup storage facility. In a sense, mirroring is a form of continual backup. Mirroring also provides some performance benefits when reading data from disks under heavy I/O loads. Windows NT-based disk mirroring implements RAID level 1.
Disk duplexing is a form of mirroring that also provides protection against controller failures¾in addition to protecting against media failures¾by the use of a different disk controller on the mirror disk.
Disk mirroring and duplexing are features of Windows NT Server. They are not supported for Windows NT Workstation. On a dual-boot computer, they are not accessible when running the MS-DOS operating system.
Windows NT-based disk mirroring or duplexing has better write performance than does Windows NT-based disk striping with parity. It also requires less system memory and does not show performance degradation during a failure.
The entry cost is also lower, since Windows NT-based disk mirroring or duplexing requires only two or more disks (compared to striping with parity, which requires three or more disks). However, mirroring provides less usable disk space (compared to striping with parity), so the cost per megabyte is higher.
Disk mirroring and duplexing are implemented by using the Microsoft Windows NT Disk Administrator program that can be started from the Administrative Tools program group. For information on setting up disk mirroring or duplexing, see your documentation for Windows NT Server, or the online Help for Disk Administrator. Also see the Windows NT Resource Kit.
Note The term mirroring is frequently used in the Windows NT Server documentation to describe both disk mirroring and duplexing.