Disk Mirroring and Duplexing

Disk mirroring protects against hard disk failure by using two partitions on different drives connected to the same disk controller. All data on the first (primary) partition is mirrored automatically onto the secondary partition. Thus if the primary disk fails, no data is lost. The partition on the secondary disk is used. Mirroring does not have to be done at the drive level. Thus, unallocated space on the primary drive can be allocated into a logical drive, as can any unallocated space on the secondary drive. With disk mirroring both partitions have the same drive letter. The user should not be aware of the mirrored disk. Disk mirroring is supported in Microsoft Windows NT Server.

Any file system can make use of disk mirroring, FAT, HPFS, or NTFS. Mirroring is not restricted to a partition of identical geometry to the primary partition (size, number of heads, cylinders, tracks, sectors). This eliminates the problem of acquiring an identical model drive to replace a failed drive when an entire drive is being mirrored. For practical purposes the mirrored partitions are usually created to be the same size as the primary partition. The mirrored partition can't be smaller. If the mirrored partition is larger than the primary, the extra space is wasted.

Disk duplexing is simply a mirrored pair with an additional adapter on the secondary drive. This provides fault tolerance for both disk and controller failure. The use of multiple adapters connecting to one drive is not supported. In addition to providing fault tolerance this also has the potential to improve performance. Duplexing is also performed at the partition level. From the standpoint of Microsoft Windows NT, there is no difference between mirroring and duplexing. It is simply a matter of where the other partition can be found.

Creating mirrored sets and duplex sets is identical. The Disk Administrator utility is used. Select the partitions that you want to duplicate and an area of free space of the same size or greater on another hard disk. This is done by selecting the partition to be mirrored, and then selecting the free space on another disk while holding down the Ctrl key. From the Disk Administrator menu, select the Fault Tolerance menu option. Then choose Establish Mirror. The Disk Administrator creates an equal-size partition in the free space for the mirror and assigns the drive letter to the set.