Windows 7 / Getting Started

---

Configuring RAID 1: Disk Mirroring

For RAID 1, disk mirroring, you configure two volumes on two drives identically. Data is written to both drives. If one drive fails, there is no data loss because the other drive contains the data. After you repair or replace the failed drive, you can restore full mirroring so that the volume is once again fault tolerant.

By using disk mirroring, you gain the advantage of redundancy. Because disk mirroring doesn't write parity information, mirrored volumes can usually offer better write performance than disk striping with parity. The key drawback, however, is that disk mirroring has a 50 percent overhead, meaning it effectively cuts the amount of storage space in half. For example, to mirror a 60-GB drive, you need another 60-GB drive. That means you use 120 GB of space to store 60 GB of information.

As with disk striping, you'll often want the mirrored disks to be on separate disk controllers. This provides redundancy for the disk controllers. If one of the disk controllers fails, the disk on the other controller is still available. When you use two separate disk controllers to duplicate data, you're using a technique known as disk duplexing rather than disk mirroring-but why mince words?

You can create a mirrored set either by using two new disks or by adding a mirror to an existing volume. As with other RAID techniques, mirroring is transparent to users. Users see the mirrored set as a single volume that they can access and use like any other drive.

Creating a Mirrored Set Using Two New Disks

To create a mirrored set using two new disks, start Disk Management. In Graphical View, right-click an area marked Unallocated on a dynamic disk, and then choose New Mirrored Volume. This starts the New Mirrored Volume Wizard. Click Next. The key difference is that you must create two identically sized volumes and these volumes must be on separate dynamic drives. The volumes can be formatted as FAT, FAT32, or NTFS. You won't be able to continue past the Selected Disks page until you've selected the two disks that you want to work with.

When you click Finish, you'll return to the main Disk Management window, and Disk Management will create the mirrored set. During the creation of the mirror, you'll see a status of Resynching. This tells you that Disk Management is creating the mirror. When this process finishes, you'll have two identical volumes. Both volumes will show the same drive letter in Disk Management, but the separation of volumes is transparent to users. Users see the mirror set as a single volume. The volume status should be listed as Healthy. This is the normal status for volumes. If the status changes, you might need to repair or resync the mirrored set.

Adding a Mirror to an Existing Volume

You can also use an existing volume to create a mirrored set. For this to work, the volume you want to mirror must be a simple volume and you must have an area of unallocated space on a second dynamic drive of equal or larger space than the existing volume. When you add a mirror onto this unallocated space, Disk Management creates a volume that is the same size and file system type as the simple volume you are mirroring. It then copies the data from the simple volume to the new volume using a process called resynching.

To add a mirror to an existing volume, start Disk Management. In Graphical View, right-click the simple volume you want to mirror, and then select Add Mirror. This displays the Add Mirror dialog box. Use the Disks list to select a location for the mirror, and then click Add Mirror. Windows Server 2008 begins the mirror creation process, and you'll see a status of Resynching on both volumes.

When the resynching is complete, you have two identical copies of the original volume. Although both volumes show the same drive letter in Disk Management, the separation of volumes is transparent to users. Users see the mirror set as a single volume.