A mirrored volume is a fault-tolerant volume that duplicates your data on two or more physical disks. A mirror provides redundancy by simultaneously writing the same data onto two or more separate mirrors (or plexes) that reside on different disks. If one of the disks fails, data continues to be written to and read from the unaffected disk or disks.

Mirrored volumes protect the data on the disk from media failure. Mirroring reduces the chance of an unrecoverable error by providing duplicate sets of data, but it also multiplies the number of disks required for data storage and the input/output (I/O) operations when writing to the disk. However, some performance gains are achieved for reading data because of I/O load balancing of requests between the plexes. A mirrored volume is about the same as a RAID-5 volume in read operations but faster in write operations.

You can create a mirrored volume with the Create Volume wizard, or you can add a mirror to an existing volume with the Add Mirror wizard. To create a mirrored volume on two or more disks, each mirror or plex of the original volume requires space that is at least the same size as the original volume. When you create a mirror, the same drive letter is used for all plexes of the volume. If you have sufficient disks available, you can create multiple mirrors for extra redundancy (up to the limit of 32 mirrors). Then, you can break off a mirror to use for backup, data analysis, or testing without losing redundancy.

Breaking off a plex of the mirrored volume does not delete the information, but it does mean that the plex that is broken off no longer mirrors information from the other plex or plexes in the mirrored volume. The broken-off plex is assigned a different drive letter than that of the original mirrored volume. It is no longer part of the mirrored volume, but it retains its other volume layout characteristics. For example, if you had a mirrored striped volume, the broken-off plex would become a striped volume.

In the case of an unrecoverable error on a plex within a mirrored volume, you need to remove the plex that is damaged with the Remove Mirror command. You then can use the Add Mirror command to create a new plex on another disk to take the place of the damaged plex.

When you want to use the space in a mirrored volume for other purposes, you can remove a plex and return the space that it used to unallocated space.

Storage Foundation lets you add a mirror to any kind of dynamic volume, except RAID-5. You can extend any mirrored volume.