Mirrored Volumes

A mirrored volume provides an identical twin of the selected volume. All data written to the mirrored volume is written to both volumes, which results in disk capacity of only 50 percent.

In this chapter, the terms original disk and original volume refer to the original volume that the data was written to, and shadow disk and shadow volume refers to the disk or volume that contains the copy.

Figure 11.2 shows a mirrored volume. To a user, only one read or write occurs to satisfy a request for data. For each user read request, Dmio.sys creates one read. For each user write request, Dmio.sys creates two writes.

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.

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.

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.

Advantages of Mirrored Volumes

Random disk read operations on mirrored volumes are more efficient than on a single volume. Dmio has the capacity to load balance read operations across the physical disks. With current SCSI technology, two disk read operations can be done simultaneously.

Recovery from a disk failure is very rapid. Mirrored volumes offer the fastest data recovery, with the least impact on system performance, because the shadow volume contains all of the data. There is no data recomputation building needed to restore the system. When you configure your boot volume on a mirrored volume, you do not have to reinstall the Windows 2000 Server to be able to restart the computer.

If a mirrored system fails, you do not need to restart the computer. The system continues to run and, if your hardware supports it, you can replace the failed drive then resynchronize the mirror. 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 a lot of I/O operations.

It is not necessary to use identical physical disks or to have the same volumes on each disk, although it is recommended that you use identical disks if you put your system volume on a mirrored volume. However, sufficient unused space on the second disk is required to create the shadow volume.

Disadvantages of Mirrored Volumes

Disk write operations are less efficient. Because data must be written to both disks, there is a slight performance penalty. However, the penalty is not 100 percent. In many situations, a user-mode application is not affected by the extra disk update.

Mirrored volumes are the least efficient in terms of space utilization. Because the data is duplicated, the space requirements for a mirrored volume are higher than for a RAID-5 volume.

For information about recovering from the failure of a mirrored volume, see "Repair, Recovery, and Restore" in this book.