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.

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.

Almost any volume can be mirrored, including the system and boot volumes. You cannot extend a mirrored volume to increase the size of the volume later. On Itanium-based computers, you cannot mirror the Extensible Firmware Interface (EFI) system partition on GUID partition table (GPT) disks.

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.

 Important

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

Operating systems on which mirrored volumes are available

Mirrored volumes are available on all computers running Windows 2000 Server, Windows 2000 Advanced Server, Windows 2000 Datacenter Server, and . Mirrored volumes are not available on computers running Windows 2000 Professional, Windows XP Home Edition, Windows XP Professional, or Windows XP 64-Bit Edition. You can, however, use a computer running Windows 2000 Professional or Windows XP Professional to create mirrored volumes on a remote computer running Windows 2000 Server, Windows 2000 Advanced Server, Windows 2000 Datacenter Server, or .

Advantages of mirrored volumes

Mirrored volumes, which use RAID-1, are slower than RAID-5 volumes in read operations but are faster in write operations. Random disk-read operations on mirrored volumes are more efficient than on a simple volume and have similar performance as striped volumes. has the capacity to load balance read operations across the physical disks. With current SCSI and Fibre Channel technology, two disk-read operations occur simultaneously.

Unlike RAID-5 volumes, a mirrored volume that loses redundancy causes the least impact on system performance. This is because the remaining disk contains all of the data; no data recomputation is needed to run the system. When you configure your boot volume on a mirrored volume, you need not reinstall to restart the computer after a disk failure.

Compared to a RAID-5 volume, a mirrored volume provides the following advantages:

Has a lower cost because it requires only two disks; a RAID-5 volume requires three or more disks.

Requires less system memory.

Provides better overall performance.

Does not degrade performance during a failure except when high-volume read operations are performed. If a single write error occurs, however, redundancy is lost.

Disadvantages of mirrored volumes

One disadvantage of mirrored volumes is that disk-write operations on mirrored volumes are less efficient because data must be written to both disks. This performance drawback is offset slightly, however, because disk-write operations on both disks can usually take place concurrently.

Another disadvantage is that performance is negatively affected when the system resynchronizes a mirrored volume. Resynchronization is the process by which a mirrored volume's mirrors are made to contain identical data. During resynchronization, performance is affected because the computer is performing many input/output (I/O) operations to copy the data.

Compared to a RAID-5 volume, a mirrored volume has the following disadvantages:

Has a higher cost per gigabyte.

Uses space less efficiently. Because the data is duplicated, the space requirements for a mirrored volume are higher than for a RAID-5 volume.

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



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