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Recovering a Mirrored Volume

A mirrored volume is provides fault tolerance by duplicating data on two disks. If one disk fails, the data on the failed disk becomes unavailable, but the system continues to operate using the unaffected disk.

If the failure of a mirrored volume did not cause any disruption in service, you can continue running in a non-fault-tolerant configuration and schedule a time to reconstruct the mirrored volume. This activity can occur during a normally scheduled maintenance period or during a less busy time. However, if you have a spare disk in the configuration, you can reconstruct the mirror immediately.



The failed disk can be replaced with any disk that is the same size or larger.

It is a good idea to use a disk as similar to the remaining disk as possible.

When you move or replace a disk that was at the end of a SCSI bus, be sure that you terminate only the disk that is now at the end of the bus.

When a member of a mirrored volume is orphaned, you need to break the mirrored volume to expose the remaining volume as a separate volume. The remaining member of the mirrored volume receives the drive letter that was assigned to the complete mirrored volume. The orphaned volume receives the next available drive letter or a new letter assigned to it.

You can then create a new mirrored volume from unused free space on another disk. When you restart the computer, the data from the working volume is copied to the new member of the mirrored volume.



In Windows NT 4.0 and earlier, mirrored volumes were known as mirror sets. Disk Management renames all mirror sets to Mirrored Volume. These mirrored volumes reside only on basic disks.

Repairing a Basic Mirrored Volume

When you follow the procedure to repair a basic mirrored volume, the status of the mirrored volume changes to Regenerating and then Healthy. If the volume does not return to the Healthy status, right-click the volume, and then click Resynchronize Mirror .



If a basic disk containing part of a mirrored volume is disconnected or fails, the status of the mirrored volume becomes Failed Redundancy and the disk status remains Online. If this happens, you can try to repair the volume.

When you repair a mirrored volume on a basic disk, Disk Management creates a new mirror on a healthy disk and then resynchronizes the new mirror.

Repairing a mirrored volume on a basic disk requires another basic disk with sufficient free space for the new mirror. If an additional disk is not available, the Repair Volume option is unavailable and you cannot repair the volume.

You must use a basic disk to repair a basic mirrored volume (mirror set). You cannot use a dynamic disk.

Replacing a Failed Mirror

If the disk containing part of the mirrored volume cannot be reactivated and the volume does not return to the Healthy status, replace the failed mirror with a new mirror on another disk.

To replace the failed mirror with a new mirror on another disk

  1. Open Disk Management.

  2. Right-click the mirror on the missing or offline disk, and then click Remove Mirror . Follow the instructions on your screen.

  3. Right-click the volume to be remirrored, and then click Add Mirror . Follow the instructions on your screen.

To replace a mirror in the mirrored volume, you need a dynamic disk with unallocated space that is at least as large as the region to repair. If you don't have a dynamic disk with enough unallocated space, the Add Mirror command is unavailable.

Breaking a Mirrored Volume

Breaking the mirrored volume results in two independent partitions or logical drives. No information is deleted, but the data is no longer redundant. Back up the volume before breaking a mirrored volume.

Deleting Mirrored Volumes on a Basic Disk

Deleting a mirrored volume deletes all the data contained in the volume as well as the partitions that make up the volume. You can delete only entire mirrored volumes.

Resynchronizing Mirrored Volumes

Resynchronize a mirrored volume when data on one disk becomes stale. For example, if one disk of a mirrored volume is disconnected, data is written to the remaining disk, but the volume is no longer fault tolerant. If you reconnect the disk, the data on the reconnected disk is stale. To make the mirrored volume fault tolerant again, resynchronize the mirrored volume to update the information on the reconnected disk.

In most cases, mirrored volumes on dynamic disks are resynchronized automatically. However, you need to use the Resynchronize Mirror command for mirrored volumes on basic disks.

Removing Mirrored Volumes

Once you remove a mirror from a mirrored volume, that mirror becomes unallocated space; the remaining mirror becomes a simple volume and is no longer fault tolerant. All data on the removed mirror is deleted.