If there is a read failure on one of the disks, Dmio reads the data from the other disk in the mirrored volume. If there is a write failure on one of the disks in the mirrored volume, the remaining disk is used for all accesses.

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.

Any volume can be mirrored, including the system and boot volumes. The disk that you select for the shadow volume does not need to be identical to the original disk in size, or in its number of tracks and cylinders. This means that you do not have to replace a failed disk with an identical model. The unused area that you select for the shadow volume cannot be smaller than the original volume. If the area that you select for the shadow volume is larger than the original, the extra space on the shadow disk can be configured as another volume.

Note

As with striped volumes, you cannot add disk space to a mirrored volume to increase the size of the volume later.

When compared to a RAID-5 volume, a mirrored volume implementation:

Has a lower entry cost because it requires only two disks, whereas a RAID-5 volume requires three or more disks, but does cost more per gigabyte.
Requires less system memory.
Provides very good overall performance.
Does not show performance degradation during a failure. However, in the event of a single write error, redundancy is lost.
Has a higher cost-per-megabyte.

Note

The term system volume refers to the disk volume containing hardware-specific files needed to start Windows 2000 (such as the files NTLDR and Boot.ini). The boot volume contains the operating system files and support files. The boot volume and the system volume can be the same volume.

A mirrored volume works well in the following situations:

When extremely high data reliability is required. A duplexed mirrored volume has the best data reliability because the entire I/O subsystem is duplicated.
When simplicity is important. Mirrored volumes are simple to understand and easy to set up.

You might not want to use a mirrored volume if cost is a critical factor. Mirrored volumes are the most expensive solution based on the cost per unit of data storage.

	Created by kamsz over 10 years ago

Microsoft 70-410 Note on Mirrored volume, created by kamsz on 19/08/2013.