Using mirrored volumes

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Using mirrored volumes

A mirrored volume is a fault-tolerant volume that provides data redundancy by using two copies, or mirrors, of the volume to duplicate the data stored on the volume. All data written to the mirrored volume is written to both mirrors, which are located on separate physical disks.

If one of the physical disks fails, the data on the failed disk becomes unavailable, but the system continues to operate using the unaffected disk. When one of the mirrors in a mirrored volume fails, you must break the mirrored volume to expose the remaining mirror as a separate volume with its own drive letter. You can then create a new mirrored volume with unused free space of equal or greater size on another disk. When creating mirrored volumes, it is best to use disks that are the same size and model and from the same manufacturer. You cannot extend a mirrored volume to increase the size of the volume later.

Because dual-write operations can degrade system performance, many mirrored volume configurations use duplexing, where each disk in the mirrored volume resides on its own disk controller. A duplexed mirrored volume has the best data reliability because the entire input/output (I/O) subsystem is duplicated. This means that if one disk controller fails, the other controller (and thus the disk on that controller) continues to operate normally. If you do not use two controllers, a failed controller makes both mirrors in a mirrored volume inaccessible until the controller is replaced.

When you mirror the system or boot volumes, you can make the configuration more fault tolerant by using a separate disk controller for each disk in the mirrored volume. This enables your computer to survive hard-disk or disk-controller failures. When creating mirrored volumes, it is best to use disks that are the same size and model, and from the same manufacturer. If you are using duplexing, it is recommended that you use identical disks and controllers, especially if you plan to mirror the system or boot volumes. For more information, see Create and test a mirrored system or boot volume. On Itanium-based computers, you cannot mirror the Extensible Firmware Interface (EFI) system partition on GUID partition table (GPT) disks. Instead, you must use the bootcfg command to clone the EFI system partition.


  • When mirroring the system volume, always test to make sure you can start the operating system from each mirror if one of the disks fails. To help prevent startup problems, always use identical disks and controllers.

You can create mirrored volumes using a combination of master boot record (MBR) and GUID partition table (GPT) disks, but the MBR cylinder-alignment restriction may cause some difficulties in creating mirrored volumes. Always mirror the MBR disk to the GPT disk to avoid cylinder-alignment difficulties.

For step-by-step procedures on how to create, break, or repair a mirrored volume, or to add or remove a mirror from an existing mirrored volume, see Manage Mirrored Volumes.

For information about which operating systems support mirrored volumes, see Partition styles.

For more information about mirrored volumes, see "Disk Management" at the Microsoft Windows Resource Kits Web site.