How to create a Mirrored Volume

In simple terms, a mirrored volume (RAID-1) writes the same data on two disks or partitions of separate disks. That is, any changes to data (or new data) made to the first disk of the mirror set are also made to its mirror disk. Mirrored volumes require at least two disks or two free partitions on separate disks. The free partitions or disks need to be of the same size, otherwise, the mirrored volume size would be equal to the smallest partition or disk.

The main advantage of mirrored volumes is disk drive redundancy. It provides availability of data in case one disk drive fails while it can mirror a system disk containing the operating system.  If a disk fails, the mirror set continues to operate on the remaining disk. The recovery operation of the mirror set depends on which disk fails if the set contains the system disk.  In general you would break/remove the mirror set, replace the faulty disk and re-create the set.

Disks participating in a mirror set needs to be Dynamic, while, the main disadvantage of mirrored volumes is the storage space requirements. For a mirrored volume of size 500GB you would need two 500GB free partitions or disks – 1GB of storage capacity in total when in fact only 500GB is available! As regards to performance, mirrored volumes may improve read transfer speeds and slow down write operations. Overall, you would not get any I/O performance benefits.

To create a mirrored volume, follow these steps with administrator privileges:

  1. Open the Disk Management console by typing disk management in the Start search text box and click Create and format hard disk partitions or from the Computer Management console found in Control Panel\All Control Panel Items\Administrative Tools
  2. Convert participating disks to Dynamic if they are set as Basic, right-click a Disk (left-hand side) and select Convert to Dynamic – if you skip this step, you are prompted to convert the disks at a later stage
  3. Right click on a new disk or unallocated space of a used disk and select New Mirrored Volume – the New Mirrored Volume Wizard starts
  4. Select the second disk – the size should be as large as the disk or partition you want to mirror
  5. The next screen shows the default assigned drive letter, you can set any available letter from the drop-down list, while, you can also create a volume without a drive letter or mount it in an empty folder
  6. In the next format partition screen, it is recommended to leave the default file system and allocation unit size settings while type in a meaningful name in the volume label text field. 
  7. The final screen is a summary of all set parameters, if no changes are required click Finish

When using the DiskPart command-line tool, first use the select disk command to select the first disk. Then, use the command add disk=<disk Id> to specify the mirror disk.