Disk Mirroring

Disk mirroring is a method that protects against hard disk failure. Any file system — including FAT, HPFS, and NTFS — can make use of disk mirroring. Disk mirroring uses 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. Instead, the partition on the secondary disk is used.

Mirroring is not restricted to a partition identical to the primary partition in size, number of tracks and cylinders, and so on. 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, though, the mirrored partitions will usually be created to be the same size as the primary partition. The mirrored partition cannot be smaller. However, if the mirrored partition is larger than the primary, the extra space is wasted.